Networking Glossary

This page has some general networking definitions.
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Definitions


A
AC-3 (Dolby Digital)
Definition: This digital surround-sound format for home audio is called Dolby Digital in theaters. It is a 5.1 format, with six separate audio tracks. AC-3 has been chosen as the official sound format for digital TV, and is already encoded on many DVDs and Laserdiscs.

ACK
Definition: When a modem receives a data packet, it sends a signal back to the sending modem. If all the data is present and correct, it sends an ACK (acknowledgement) signal, which acts as a request for the next data packet. If the modem didn't get all the data, it sends back a negative acknowledgement, or NAK

Also Known As: Acknowledgement

ACPI
Definition: This power management standard, proposed by Microsoft, Intel, and Toshiba, lets the PC control power to peripherals like CD-ROMs and printers, as well as consumer devices hooked up to the PC. Peripherals can also use ACPI to turn on the PC. For example: you could insert a CD-ROM into a drive, and the computer would automatically boot up.

Also Known As: Advanced Configuration and Power Interface

ACTIVE CHANNELS
Definition: An Active Channel is what Microsoft calls a Web site that has been enabled for push delivery to Internet Explorer 4.0 browsers. To create a channel, developers write and upload a CDF (channel definition format) file to their Web site; new content is delivered to users automatically when the site is updated. Developers and subscribers can control the update frequency; which channels, subchannels, and items (sections) are subscribed to; and other channel characteristics. Most Active Channels use dynamic HTML (DHTML) and other effects to spice up content and make it more interactive.

ACTIVE MATRIX
Definition: Flat-panel displays for laptop computers are created by laying diodes over a superfine grid of wires. The diodes are activated by applying current to various points around the grid. Connect enough of these dots and you get an image. Active-matrix displays use transistors to keep their diodes in an on or off state, unlike their passive-matrix cousins, which rely on the diodes' persistence. As a result, active-matrix displays are brighter and produce better color than passive-matrix displays. However, the additional technology required to build active displays also makes them far more expensive.

ACTIVE X
Definition: This set of technologies from Microsoft provides tools for linking desktop applications to the World Wide Web. Using a variety of programming tools--including Java, Visual Basic, and C++--developers can create interactive Web content. For instance, ActiveX technology can allow users to view Word and Excel documents directly in a browser.

ADAPTER
Definition: A network adapter provides a hardware interface between a computer and a network. The term was popularized originally by Ethernet add-in cards for PCs. These devices plug into the system bus of the PC and include jacks for network cables. Nowadays, most people use "network adapter" in a broader sense today to include PCMCIA devices (also know as "credit card" or "PC Card" adapters) and analogous wireless devices.
Several vendors make network adapter products. Many of these products come pre-installed as part of a total computer system configuration, so consumers may never see them. The main differences between the products are important to keep in mind, however: supported speeds (typically 10 Mbps, 100 Mbps, or both) the option of an integrated modem (especially on PCMCIA devices) the vendor's manufacturing quality.

Also Known As: network card

ADDRESS
Definition: A network address serves as a "unique" identifier for a computer on a network. Computers can determine the addresses of other computers on the network and use these addresses to send messages to each other. One of the best known form of network addressing is the Internet Protocol (IP) address. IP addresses consist of four bytes (32 bits) that uniquely identify all computers on the public Internet. Another popular form of address is the Media Access Control or MAC address. MAC addresses are six bytes (48 bits) that manufacturers of network adapters burn into their products to uniquely identify them.

Also Known As: Network Address

ADO
Definition: A Microsoft technology that provides a unified way to access different data sources with a single data model. By providing a single data model, ADO lets you access data in the same way whether it exists in a database, a spreadsheet, and even text files.

Also Known As: ActiveX Data Objects

ADPCM
Definition: This method of encoding sound data files takes up less storage space than the regular PCM format used by WAV and AIFF files--and CD audio for that matter. ADPCM comes in more than one flavor: IMA's ADPCM, for example, is used on the Sony Mini Disc to cram more data onto a smaller platter; Microsoft ADPCM is used as part of Windows 95's canon of audio codecs.

Also Known As: Adaptive Differential Pulse Code Modulation

ADSL
Definition: ADSL is a form of Digital Subscriber Line (DSL) service that provides greater bandwidth for so-called downstream (from provider to consumer) traffic at the expense of lesser upstream (from consumer to provider) bandwidth. ADSL takes advantage of the typical pattern of network access on the Web by home users who frequently download large amounts of Web site data but upload relatively small amounts of data. In other respects, ADSL possesses all of the characteristics one associates with DSL, including an "always on" combination of voice and data services, availability limited by physical distance, and high speed access compared to analog modems. ADSL is technically capable of up to 6 Mbps (roughly 6000 Kbps), but the service customers actually receive generally performs at 2 Mbps or lower for downloads and 512 Kbps for uploads.

Also Known As: Asymmetric Digital Subscriber Line

AGP
Definition: A dedicated graphics bus slot on PC motherboards. Designed by Intel, AGP operates at 66 MHz and transfers data at a rate up to 528 MB/sec. By comparison, the PCI slot that most graphics cards currently use runs at 33 MHz and has a maximum transfer rate of 132 MB/sec. AGP's greater bandwidth will allow game and 3D application developers to store and retrieve larger, more realistic textures in system memory rather than video memory, without incurring a dramatic performance hit.

Also Known As: Accelerated Graphics Port

AIFF
Definition: This audio file format was developed by Apple Computer for storing high-quality sampled audio and musical instrument information. It is also used by Silicon Graphics and in several professional audio packages. Played by a variety of downloadable software on both the PC and the Mac.

Also Known As: Audio Interchange File Format

ALPHA CHANNEL
Definition: In computer graphics, each pixel has three channels of color information--red, green, and blue--in various bit depths. In 24-bit display graphics adapters, there are 8 bits per color per pixel, but when the card has a 32-bit bus, the additional 8 bits are used as an alpha channel to control the color information of the other 24 bits of color.

ALTIVORE
Definition: Altivore is an open-source network sniffer developed by Network ICE. Altivore copies or clones the FBI's infamous Carnivore system. Network ICE created Altivore to educate the public on the inner workings of Carnivore and the privacy and security issues involved with such systems. Altivore snoops a network to monitor email traffic (both headers and content), requests to Web and FTP servers, and IP address discovery and packet sniffing of that address.

AMPLITUDE
Definition: Like any wave, analog signals contain peaks and valleys. By measuring the difference between the peaks and the valleys and then dividing by a factor of two, we get the amplitude--or strength--of the wave. The larger the amplitude, the stronger the wave.

ANALOG
Definition: The traditional method of modulating radio signals so that they can carry information. Amplitude modulation (AM) and frequency modulation (FM) are the two most common methods of analog modulation. Today, most U.S. cellular systems carry phone conversations using analog; the transition to digital transmissions is happening slowly.

ANALOG CONTROLS
Definition: Analog controls are dials, knobs, and similar mechanisms that you find on monitors and other devices. They have a potentially infinite number of settings that cannot be stored in a profile and recalled--which means that a monitor's analog controls require readjustment whenever you change the video mode (resolution or refresh rate).

ANONYMOUS FTP
Definition: Part of the Internet's appeal is the huge number of files available for download. Anonymous File Transfer Protocol (FTP) enables anyone to download files from a properly configured FTP server. This allows users without accounts to access files by entering the username anonymous, along with their email address as a password. Because anyone can access files that have been set up for anonymous FTP, Webmasters need to be careful to use it only for files meant for public consumption.

ANONYMOUS REMAILER
Definition: An anonymous remailer is a computer that strips away identifying information (such as your email address) before passing your message on to an email address or a newsgroup. When you send mail using a remailer, the From: field of your message becomes a made-up address like 12xyx@remailer.com. There are actually two kinds of services called anonymous remailers. The first is truly anonymous: no one anywhere knows your identity. With the second, called pseudo-anonymous or sometimes pseudonymous, the owner of the service knows your identity and can be forced in a court of law to reveal it. Most truly anonymous services are free (it's difficult to bill an unknown, unnamed client), but they often require some skill and effort to use. Many pseudo-anonymous services charge a fee, but are more user-friendly.

APACHE
Definition: Apache is generally recognized as the world's most popular Web server. Originally designed for UNIX® servers, it has been ported to Windows and other operating systems (although some features may be unavailable and performance may be slower compared to UNIX). The name "Apache" derives from the word "patchy" that the developers used to describe early versions of the software. Apache supports a full range of features -- including CGI, SSL, and virtual domains -- as well as plug-in modules for extensibility. It is reliable, free, and relatively easy to configure.

Also Known As: "a patchy"

API
Definition: An API allows programmers to access the functionality of a pre-built software module through well-defined data structures and subroutine calls. Although programmers often define APIs for private internal code, network APIs typically are the public entry points to libraries that hide low-level details of computer networking. Traditionally the most popular networking APIs have accompanied socket libraries. Berkeley sockets and Windows Sockets (Winsock) APIs have seen widespread use for many years. More recently, Java network APIs such as servlets have also grown in popularity.

Also Known As: Application Programming Interface

APIPA
Definition: A feature of Microsoft Windows, APIPA allows a computer configured as a DHCP client to obtain an IP address even in cases where the DHCP server cannot provide one. APIPA functions as a DHCP server failover mechanism as follows. If the client cannot obtain an address from the server, it obtains an address from the built-in, reserved address range of 169.254.0.1 through 169.254.255.254. The client verifies this address is not already in use on the LAN using ARP. Whenever the DHCP server become able to service requests, the client updates its address information accordingly. The 169.254.xxx.xxx address range is reserved for private nodes, not for computers directly connected to the Internet. In APIPA, all nodes in this range utilize the same default network mask, 255.255.0.0, and thus all reside on the same subnet. With these restrictions, APIPA works well only on home or small business networks. Though part of Windows 98, Windows ME, Windows 2000, and newer flavors, APIPA does not exist in Windows NT. APIPA is automatically enabled for all DHCP clients in Windows unless the computer's Registry is modified to disable it. APIPA can be disabled for each network adapter individually or on the entire computer.

Also Known As: Automatic Private IP Addressing; AutoNet

ARP
Definition: ARP converts an IP address to its corresponding physical network address. It is a low-level protocol (at layer 2 in the OSI model) usually implemented in the device drivers of network operating systems. ARP is most commonly seen on Ethernet networks, but ARP has also been implemented for ATM, Token Ring, and other physical networks. The first RFC discussing ARP (for Ethernet) was RFC 826. Ethernet network adapters are produced with a physical address (called the Media Access Control or MAC address) embedded in the hardware. Manufacturers take care to ensure these 6-byte addresses are unique, and Ethernet relies on these unique identifiers for frame delivery. When an IP packet arrives at a network gateway, the gateway needs to convert the destination IP address to the appropriate MAC address so that it can be delivered over Ethernet. Some IP-to-MAC address mappings are maintained in an ARP cache, but if the given IP address does not appear there, the gateway will send an ARP request that is broadcast on the local subnet. The host with the given IP address sends an ARP reply to the gateway, who in turn delivers the packet (and updates its cache).

Also Known As: Address Resolution Protocol

ARTIFACT
Definition: In all kinds of computer graphics, including any display on a monitor, artifacts are things you don't want to see. They fall into many categories (such as speckles in scanned pictures), but they all have one thing in common: they are chunks of stray pixels that don't belong in the image.

ASCII
Definition: Bland, unformatted text files are best saved in ASCII (pronounced "askee") format. But ASCII is more than a text file format--it's a standard developed by the American National Standards Institute (ANSI) to define how computers write and read characters. The ASCII set of 128 characters includes letters, numbers, punctuation, and control codes (such as a character that marks the end of a line). Each letter or other character is represented by a number: an uppercase A, for example, is the number 65, and a lowercase z is the number 122. Most operating systems use the ASCII standard, except for Windows NT, which uses the suitably larger and newer Unicode standard.

Also Known As: American Standard Code for Information Interchange

ASCEND COMPRESSION
Definition: Ascend uses a proprietary data compression implementation, based on STAC Incorporated's standard, with many of its router products. This allows products that support Ascend's STAC compression to achieve greater throughput over Internet connections

ASSYMETRIC
Definition: In a symmetric network, all devices can transmit and receive data at equal rates. Asymmetric networks support more bandwidth in one direction than the other. For example, symmetric DSL offers customers identical bandwidth for both downloads and uploads. Asymmetric DSL offers more bandwidth for customer downloads at the cost of less bandwidth for uploads. A more general but less common definition of network symmetry involves access to resources. In this sense, all devices on a symmetric network share equal access to resources or the same capabilities. Asymmetric networks divide at least some resources unequally between devices. Examples: Pure peer-to-peer networks like Gnutella implement perfect symmetry: all computers on the Gnutella network have equal opportunity to discover, publish, or receive content. A hybrid peer-to-peer network like Napster is technically asymmetric because it utilizes central servers for user registration and file catalogs. However, from the point of view of clients, Napster is effectively symmetric because all customers share equivalent functionality.

ATAPI
Definition: The Enhanced IDE standard by itself works wonders for hooking up disk drives to PCs. But that's not enough. People want to attach tape drives and CD-ROMs to the same controller. Thanks to EIDE's support for the ATAPI standard, they can. Using ATAPI-capable hardware and software drivers, you can mix and match different types of drives on the same EIDE controller, which is both a convenience and a savings in hardware costs.

Also Known As: AT Attachment Packet Interface

AT COMMANDS
Definition: AT is a contraction of attention, a command used to program SmartModems from Hayes Microcomputer Products. AT commands program a variety of modem hardware settings and were adopted by other modem manufacturers who wanted to market their wares with the coveted phrase Hayes-compatible. At one time, you couldn't call yourself an online aficionado if you didn't know that ATL0 turned your modem speaker way down and ATM0 turned it off. Now the commands are usually hidden under a menu option in your communication software.

ATM
Definition: ATM is a high-speed networking standard designed to support both voice and data communications. ATM can support speeds ATM operates at the data link layer over either fiber or twisted-pair cable. ATM differs from more common data link technologies like Ethernet in several ways. ATM does not involve routing for example. Hardware devices known as ATM switches establish point-to-point connections between endpoints and data flows directly from source to destination. Instead of using variable-length packets, ATM utilizes fixed-sized cells. ATM cells are 53 bytes in length, that includes 48 bytes of data and 5 bytes of header information. The performance of ATM is often expressed in the form of OC (Optical Carrier) levels, written as "OC-xxx." Performance levels as high as 10 Gbps (OC-192) are technically feasible with ATM. More common performance levels for ATM are 155 Mbps (OC-3) and 622 Mbps (OC-12). ATM is designed to support easier bandwidth management. Without routing and with fixed-size cells, one can much more easily monitor and control bandwidth under ATM than under Ethernet, for example. The high cost of ATM relative to Ethernet is one factor that has limited its adoption to "backbone" and other high-performance applications.

Also Known As: Asynchronous Transfer Mode

AUP
Definition: Many public network services have an AUP. The AUP is a formal or informal document that defines the intended uses of the network, unacceptable uses, and the consequences for non-compliance. One often encounters an AUP when registering for a Web-based service or when working on a corporate intranet. A good AUP will include provisions for network etiquette, limits on the use of network resources if any, and clear indications of the level of privacy a member on the network should expect. The best AUPs incorporate "what if" scenarios that illustrate the usefulness of the policy in real-world terms.

Also Known As: Acceptable Use Policy, Appropriate Use Policy

AUTHENTICATION
Definition: Authentication ensures that digital data transmissions are delivered to the intended receiver. Authentication also assures the receiver of the integrity of the message and its source (where or whom it came from). The simplest form of authentication requires a username and password to gain access to a particular account. But authentication protocols can also be based on secret-key encryption, such as DES, or on public-key systems using digital signatures.

AUTO ANSWER
Definition: A modem indicator light that tells you the modem is ready to pick up the phone, so long as there's a communication program running and prepared to handle the call. The light is often not shining even when the modem is prepared to take a call, so it's not a useful light.

AUTOSENSING
Definition: Network adapters that support both traditional and Fast Ethernet -- so-called 10/100 adapters -- choose the speed at which they run through a procedure called autosensing. Autosensing involves probing the capability of the network using low-level signalling techniques to select compatible Ethernet speeds. Autosensing was developed to make the migration from traditional Ethernet to Fast Ethernet products easier. Autosensing is a feature of 10/100 hubs, switches, and NICs. When first connected, these devices automatically exchange information with each other to agree on a common setting. The devices run at 100 Mbps if the network supports it, otherwise they drop down to 10 Mbps to ensure a "lowest common denominator" of performance. Many hubs and switches are capable of autosensing on a port-by-port basis: in this case, some computers on the network may be communicating at 10 Mbps and others at 100 Mbps. 10/100 products often incorporate two LEDs of different colors to indicate the speed setting that is currently active. Also Known As: autonegotiation


B

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BANDWIDTH
Definition: Bandwidth in computer networking refers to the data rate supported by a network connection or interface. One most commonly expresses bandwidth in terms of bytes per second (bps). The term comes from the field of electrical engineering, where bandwidth represents the total distance or range between the highest and lowest signals on the communication channel (band). Bandwidth represents the capacity of the connection. The greater the capacity, the more likely that greater performance will follow, though overall performance also depends on other factors, such as latency. Examples: A V.90 supports a maximum theoretical bandwidth of 56 Kbps. Fast Ethernet supports a theoretical maximum bandwidth of 100 Mbps.

Also Known As: throughput

BASIC
Definition: beginner's all-purpose symbolic instruction code This standard, high-level family of programming languages is simple to learn but creates programs that are typically slow to use. Many types of BASIC are available--your package might be called Turbo, Quick, or Visual Basic. The language is not generally used for industrial-strength applications, although Visual Basic has spawned a lot of shareware programs--and even some commercial apps such as MicroHelp's Uninstaller.

Also Known As: Beginner's All-purpose Symbolic Instruction Code

BAUD
Definition: A baud is a unit of measure for analog signaling. At a minimum, one baud corresponds to one bit per second (bps) although at higher signaling speeds, multiple bits can be transfered in a single baud. The term “baud” was advertised in the early days of modems, but it is no longer relevant in mainstream computer networking terminology.

Also Known As: BPS

B CHANNEL
Definition: This wire used in ISDN service can carry up to 64 kbps of data when operating at full capacity. It is a clear-channel "pipe" that carries voice, data, or video over ISDN. The Basic Rate Interface (BRI) service offered to home users has two B channels.

Also Known As: Bearer Channel

BPS
Definition: Network performance has traditionally been measured in the units of bits per second (bps). Not too many years ago, a typical dial-up network connection performed at 9600 bps. As networks have greatly improved in performance, rates are usually now specified in Kbps (thousands of bps) or Mbps (millions of bps). The unit “Bps” refers to bytes per second. Traditionally units of Bps have been avoided in networking as not all computer architectures implemented a byte with the same number of bits (some used four bits, most used eight bits, and a few used neither of these). As eight-bit byte architectures have become prevalent today, one can generally convert from Bps down to bps safely by multiplying by a factor of eight. However, some people use Bps when they really mean bps and it is important to be clear on this point.

Also Known As: bytes per second, bytes/sec, bytes/s

BINARY
Definition: A numeric system that represents all numbers using only two digits: 1 and 0. Binary is used by computers because each binary digit can be represented by a single digital switch set to either on or off. Each of these digital switches is called a bit. An 8-bit computer processor represents data using sets of 8 bits, a 16-bit processor uses 16 bits, and so on. The more bits a computer can process at once, the larger the numbers it can work with.

BIOS
Definition: The BIOS is what's coded into a PC's ROM to provide the basic instructions for controlling system hardware. The operating system and application programs both directly access BIOS routines to provide better compatibility for such functions as screen display. Some makers of add-in boards such as graphics accelerator cards provide their own BIOS modules that work in conjunction with (or replace) the BIOS on the system's motherboard.

Also Known As: basic input/output system

BROADBAND
Definition: The term "broadband" refers to any type of transmission technique that carries several data channels over a common wire. DSL services, for example, combine separate voice and data channels over a single telephone line -- voice fills the low end of the frequency spectrum and data fills the high end.

Examples: Both cable TV and DSL service rely on broadband transmission techniques.


C
CABLE MODEM
Definition: A cable modem connects a home computer (or network of home computers) to residential cable TV service. Like the television "set top" box, cable modems are usually supplied by the service provider and not a piece of equipment individuals need to shop for on their own. Most cable modems supply a 10 Mbps Ethernet connection for the home LAN, although cable modem service rarely if ever performs at those speeds. The performance of a cable modem Internet connection can vary depending on the utilization of the shared cable line in that neighborhood, but typical data rates range from 300 Kbps to 1500 Kbps.

Also Known As: broadband modem

CARNIVORE
Definition: Carnivore is a "network diagnostic tool" created by the U.S. Federal Bureau of Investigation (FBI) to assist in crime investigations. It is a secure computer platform running Windows/NT or Windows 2000 and application software developed by the FBI. The technology of Carnivore does not differ substantially from other network sniffers that have been commercially available for years. Carnivore has drawn special attention, however, because of the large amount of Internet traffic it is capable of capturing. The FBI typically installs Carnivore in an ISP data center when investigating individuals suspected of federal crimes such as terrorism. Carnivore snoops essentially all data flowing through the network and saves the bits that fit a specific profile -- email sent or received from a particular user name, for example, or all data sent to Web sites from a particular IP address. Although the data of many other uninvolved people on the Net may flow through the Carnivore system, the FBI claims that the privacy of this data will not be compromised.

Also Known As: Omnivore

CAT 5
Definition: CAT5 is an Ethernet cable standard defined by the Electronic Industries Association and Telecommunications Industry Association (commonly known as EIA/TIA). CAT5 is the 5th generation of twisted pair cabling and the most popular of all twisted pair cables in use today. CAT5 cable contains four pairs of copper wire. It supports Fast (100 Mbps) Ethernet and comparable alternatives such as ATM. As with all other types of twisted pair EIA/TIA cabling, CAT5 cable runs are limited to a maximum recommended run rate of 100m (328 feet). Although CAT5 cable usually contains four pairs of copper wire, Fast Ethernet communications only utilize two pairs. A new specification for CAT5, CAT5 enhanced (CAT5e), supports short-run Gigabit Ethernet (1000 Mbps) networking by utilizing all four wire pairs and is backward-compatible with ordinary CAT5. Twisted pair cable like CAT5 comes in two main varieties, solid and stranded. Solid CAT5 supports longer runs, whereas stranded CAT5 is more flexible and better suited for "patch" cabling.

Also Known As: Category 5

CGI
Definition: Common Gateway Interface. CGI usually means the program that is run on the server to add dynamic actions to the Web site.

CLUSTER
Definition: The word "cluster" is used broadly in computer networking to refer to a number of different implementations of shared computing resources. Typically, a cluster integrates the resources of two or more computing devices (that could otherwise function separately) together for some common purpose. A Web server farm (a collection of networked Web servers, each with access to content on the same site) function as a cluster conceptually. However, purists may debate the classification of a server farm as a cluster, depending on the details of the hardware and software configuration. It is important to recognize that network clustering involves a long past history of research and development with many offshoots and variations.

Also Known As: Farm, Network of Workstations (NOW), distributed system, parallel system

CRASH
Definition: The term "crash" refers to a critical failure in a computer, network device, or software running on these (such as a network operating system or application). Crashes often occur with little or no warning. As evidenced by the number of different synonyms employed over the years, crashes have a long and colorful history in computer networking. A crash can manifest itself in many different ways. The computer may "freeze" or "hang" indefinitely when a crash occurs, or it may display any number of obscure error messages. In Windows operating systems, a crash usually causes the infamous "blue screen of death" to appear. To recover from a crash, often a person must power down and restart their computer (as in the case of blue screens) or their network device(s).

Also Known As: crash and burn, fatal error, bomb, blow-up, core dump, blue screen of death (BSOD), ABEND


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D
DCC
Definition: A Windows 95 feature that enables two computers to be connected via a serial or parallel cable. Once connected, the two computers function as if they were on a local-area network (LAN). Either computer can access files on the other computer. But because DCC does not require network interface cards (NICs), it is less expensive and simpler. The limitations are that it can connect only two PCs, and the data transfer rate is slower than with a true LAN.

Also Known As: Direct Cable Connection

DHCP
Definition: DHCP allows a computer to join an IP-based network without the need for it to have a pre-configured IP address. The protocol assigns unique IP addresses to devices, and it releases and renews these addresses as devices leave and re-join the network. Many Internet Service Providers (ISPs) use DHCP to allow customers to join the Internet with minimum effort. DHCP systems require a server set up with the appropriate configuration parameters for the given network. These parameters include the range or pool of available IP addresses, the correct subnet masks, and gateway and name server addresses. Devices running DHCP client software can then automatically retrieve these settings as needed. Using DHCP on a network means that system administrators do not need to configure these parameters individually for each device joining the network.

Also Known As: Dynamic Host Configuration Protocol

DNS
Definition: The DNS translates Internet domain and host names to IP addresses. DNS implements a distributed database to store name and address information for all public hosts on the Net. DNS assumes IP addresses do not change (i.e., are statically assigned rather than dynamically assigned). The DNS database resides on a hierarchy of dedicated servers. When visiting a Web site or other device on the Net, a piece of software called the DNS resolver (usually built into the network operating system) first contacts a DNS server to determine the server's IP address. If the DNS server does not contain the needed mapping, it will in turn forward the request to a DNS server at the next higher level in the hierarchy. After potentially several messages are sent within the DNS hierarchy, the IP address for the given host eventually is delivered to the resolver. DNS also includes support for caching requests and for redundancy. Most network operating systems allow one to enter the IP addresses of primary, secondary, and tertiary DNS servers, each of which can service initial requests from clients. Many ISPs maintain their own DNS servers and use DHCP to automatically assign the addresses of these servers to dial-in clients, so most home users need not care about this detail. Also Known As: Domain Name System



DOWNSTREAM?UPSTREAM
Definition: From the user's perspective, upstream network traffic flows away from the local computer toward the remote destination. Conversely, downstream traffic flows to the user's computer. Traffic on most networks flows in both upstream and downstream directions simultaneously, and often when data flows in one direction, network protocols often send control instructions (generally invisible to the user) in the opposite direction. One way to generate upstream traffic is to upload files to a server or send an email message. Conversely, downloading files and receiving email generate downstream traffic. Typical Internet users create much more downstream than upstream traffic. Examples: The Web browser sends HTTP requests upstream to the Web server, and the server replies with downstream data usually in the form of HTML pages. Asymmetric DSL (ADSL) services provides less bandwidth in the upstream direction in order to reserve more bandwidth for downstream traffic.

DSL
Definition: DSL provides high-speed networking over ordinary phone lines using digital modem technology. It integrates voice and data functionality, bringing the "always on" feature of the telephone to the computer network. The technology theoretically supports data rates of 8,448 Kbps (8.448 Mbps), although typical rates vary from 1,544 Kbps to 512 Kbps and sometimes as low as 90 Kbps. DSL services target home and small business customers. However, the service only works over a limited physical distance and remains unavailable in many residential areas. The term "xDSL" generally refers to the complete family of related DSL technologies -- ADSL, SDSL, VDSL, and so on -- rather than any one specific implementation.

Also Known As: Digital Subscriber Line, Digital Subscriber Loop, xDSL


E
ECHELON
Definition: Echelon is a worldwide network monitoring system developed by the USA and UK starting in the 1940s. Allegedly, Echelon "five years ago was bigger than the Internet" and remains active today, monitoring phone calls and data communications of prominent international people. Although the FBI's Carnivore system tends to grab most of the headlines, Echelon has the potential to be a much larger privacy concern. The technology behind Echelon remains shrouded in secrecy, but a number of articles have been produced recently that purport to reveal details of the system. Some claim the system includes 120 American satellites and various ground stations, advanced voice recognition software, and specialized "dictionary computers" capable of quickly searching through large amounts of data for keywords.

Also Known As: Big Brother

ETHERNET
Definition: Ethernet is a physical and data link layer technology for LAN networking. When it first began to be widely deployed in the 1980s, Ethernet supported a maximum theoretical data rate of 10 megabits per second (Mbps). More recently, Fast Ethernet standards have extended traditional Ethernet technology to 100 Mbps peak, and Gigabit Ethernet technology extends performance up to 1000 Mbps. Higher level network protocols like IP use Ethernet as their transmission medium. Data travels over Ethernet in the form of frames, and collisions can occur when multiple devices on the line attempt to transmit simultaneously. The run length of Ethernet cables is limited (to roughly 100 meters), but various special-purpose devices exist that make Ethernet a cost-effective option for networking an entire large school or office building.


F
FAST ETHERNET
Definition: Fast Ethernet supports a maximum data rate of 100 Mbps. It is so named because original Ethernet technology supported only 10 Mbps. Fast Ethernet began to be widely deployed in the mid-1990s as the need for greater LAN performance became critical to universities and businesses. A key element of Fast Ethernet's success was its ability to coexist with existing network installations. Today, many network adapters support both traditional and Fast Ethernet. These so-called "10/100" adapters can usually sense the speed of the line automatically and adjust accordingly. Just as Fast Ethernet improved on traditional Ethernet, Gigabit Ethernet improves on Fast Ethernet, offering rates up to 1000 Mbps instead of 100 Mbps.

Also Known As: 100 Mbps Ethernet

FAQ
Definition: The FAQ is a time-honored tradition on the Internet. It explains the sometimes boring details of technology in an easy-to-read question and answer format. FAQs are written by individuals, volunteer groups, and corporations. FAQs exist for nearly all basic computer networking technologies. A typical FAQ entry will appear as follows: Q: Who created Napster, the MP3 music sharing system? A: Shawn Fanning conceived and implemented Napster while a college student in Massachusetts. The name "Napster" was a nickname his friends gave him because of his hair style.

Also Known As: Frequently Asked Questions, Frequently Answered Questions

FIREWALL
Definition: A firewall protects a computer network from unauthorized access. Firewalls may be hardware devices, software programs, or a combination of the two. A firewall typically guards an internal network against malicious access from the outside; however, firewalls may also be configured to limit access to the outside from internal users. Perhaps the most familiar form of Internet firewall is a proxy server. Proxy servers act as an intermediary between internal and external computers by receiving and selectively blocking data packets at the network boundary. They also provide an extra measure of safety by hiding internal LAN addresses from the outside. In a proxy server environment, network requests from multiple clients appear to the outsider as all coming from the same proxy server address.

Also Known As: proxy, gateway

FIREWIRE
Definition: IEEE 1394 is a standard interface for connecting storage, digital video and many other devices to your computer. Developed by Apple computer and trademarked with the name Firewire, IEEE 1394 offers data bandwidth up to 40 megabytes/second. One IEEE 1394 port can support up to 63 devices, with the assistance of one or more IEEE 1394 hubs. The IEEE 1394 standard has special modes for transferring video and other "synchronous" information, making it ideal for communicating with video cameras. The version of IEEE 1394 developed by Sony for its video equipment is called i.LINK. Pronunciation: i-triple-e 1394 •

Also Known As: Firewire, i.LINK

FTP
Definition: FTP allows one to transfer files between computers on the Internet. Technically, FTP is a simple network protocol based on IP, but many also use the term "FTP" to refer to this type of file sharing service. The FTP service is based on a client/server architecture. An FTP client program initiates a connection to a remote computer running FTP server software. After the connection is established, the client can choose to send and/or receive copies of files, singly or in groups. To connect to an FTP server, a client generally requires a username and password as set by the administrator of the server. Many public FTP archives follow a special convention for that accepts a username of "anonymous." FTP clients are included with most network operating systems, but most operating system clients (such as FTP.EXE on Windows) support a relatively unfriendly command-line interface. Many freeware and shareware third-party FTP clients have been developed that support graphic user interfaces (GUIs) and additional convenience features. In either command-line or graphic interfaces, FTP clients identify the server either by its IP address (such as 192.168.0.1) or by its host name (such as ftp.about.com). The FTP protocol supports two modes of data transfer: plain text (ASCII), and binary. The mode an FTP client uses must generally be configured by the end user. The mode usually defaults to plain text. The most common error one makes in using FTP occurs when attempting to transfer a binary file (such as a program or music file) while in text mode. A copy of the file is made, but this copy will often be unusable. When working with FTP clients and files, learn to use the transfer mode properly.

Also Known As: File Transfer Protocol


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G
GATEWAY
Definition: "Gateway" is a generic term for an internetworking system (a system that joins two networks together). Gateways can be implemented completely in software, completely in hardware, or as a combination of the two. Depending on their implementation, gateways can operate at any level of the OSI model from application protocols to low-level signaling. Because a gateway by definition appears at the edge of a network, related functionality like firewalling tends to exist at the same location.

GBPS
Definition: One kilobit per second (Kbps) equals 1000 bits per second (bps). Kbps is also written as “kbps” that carries the same meaning. Likewise, one megabit per second (Mbps) equals one million bps and one Gigabit equals one billion bps. Network performance is best measured in bps, but sometimes numbers are given in bytes per second (Bps). Then, one KBps equals one kilobyte per second, one MBps equals one megabyte per second, and GBps equals one gigabyte per second. Many times people write KBps, for example, when they mean Kbps, and it is important to be clear on this distinction. Examples: Traditional Ethernet supports data rates up to 10 Mbps and Fast Ethernet 100 Mbps. Gigabit Ethernet supports 1000 Mbps or 1 Gbps.

Also Known As: Kb/sec, Kb/s, Mb/sec, Mb/s, Gb/sec, Gb/s

GIGABIT ETHERNET
Definition: Gigabit Ethernet supports a maximum data rate of 1000 Mbps. At one time, it was believed that these speeds required fiber optic or copper cabling, but Gigabit Ethernet has now successfully been implemented on CAT5 cable. The migration from 100 Mbps Fast Ethernet to Gigabit will likely not be as smooth or rapid as was the migration from traditional Ethernet to Fast. Much more legacy Ethernet technology (both 10 and 100 Mbps varieties) exists now than it did even five years ago. Today, Gigabit can only be found mainly in research institutions. A decrease in cost, increase in demand, and improvements in other aspects of LAN technology will be required to drive the adoption of Gigabit Ethernet.

Also Known As: 1000 Mbps Ethernet

GIGABYTE
Definition: A kilobyte (KB) equals 1024 (or 210) bytes. Likewise, a megabyte (MB) equals 1024 KB or 220 bytes and a gigabyte (GB) equals 1024 MB or 230 bytes. The meaning of the words kilobyte, megabyte, and gigabyte change when they are used in the context of network data rates. A rate of one kilobyte per second (KBps) equals 1000 (not 1024) bytes per second; one megabyte per second (MBps) equals one million (106, not 220) bytes per second; one gigabyte per second (GBps) equals one billion (109, not 230) bytes per second. To avoid some of this confusion, networkers typically measure data rates in bits per second (bps) rather than bytes per second (Bps) and use the terms kilobyte, megabyte, and gigabyte only when referring to disk space. Examples: "This eight gigabyte disk only has 200 megabytes of free space left."

Also Known As: K, KB, M, MB, "meg," G, GB, "gig"

GROOVE
Definition: The Groove system allows people on the Internet to meet and work together. Groove includes applications such as chat, file sharing, and bulletin boards. It also includes programming libraries and APIs that allow software developers to build their own applications or extend the existing ones. Groove is a distributed system based on peer-to-peer (P2P) design concepts. It uses popular technologies like COM and XML, encryption, and a wide variety of network protocols. To work with Groove, customers download and install the software package that includes a user interface and sophisticated built-in networking capability. The developers of Groove formerly worked on the Lotus Notes groupware system and applied their learnings from that project to create a tool that should appeal to a broad audience.

Also Known As: Notes, the Next Generation

GROUPWARE
Definition: Groupware is software designed to improve the productivity of individuals with common goals or interests. Groupware relies on computer networking to open communications channels among people and to share data. Traditional groupware systems like Lotus Notes were designed for corporate intranets and other LANs to support collaborative work. They essentially combined the functionality of email, messaging and conferencing, and document management systems. More recently, groupware applications like Groove have been designed with similar functionality for the Internet.

Also Known As: collaboration software


H
HDSL
Definition: HDSL technology was developed in the early 1990s, making it one of the oldest forms of DSL. HDSL service provides equal bandwidth for both downloads and uploads, offering data rates up to 2,048 Kbps. Note that HDSL requires multiple phone lines to accomplish this. Like other forms of DSL, HDSL features "always on" combined voice and data services. HDSL enjoyed some popularity in years past, but the technology has effectively been replaced by alternatives like SDSL that only require one phone line and offer similar performance and reach.

Also Known As: High-data-rate DSL, High-speed DSL

HOP
Definition: In computer networking, a hop represents one portion of the path between source and destination. When communicating over the Internet, for example, data passes through a number of intermediate devices (like routers) rather than flowing directly over a single wire. Each such device causes data to "hop" between one point-to-point network connection and another. In networking, the hop count represents the total number of devices a given piece of data (packet) passes through. Generally speaking, the more hops data must traverse to reach their destination, the greater the transmission delay incurred. Network utilities like ping can be used to determine the hop count to a specific destination. Ping generates packets that include a field reserved for the hop count. Each time a capable device receives these packets, that device modifies the packet, incrementing the hop count by one. In addition, the device compares the hop count against a predetermined limit and discards the packet if its hop count is too high. This prevents packets from endlessly bouncing around the network due to routing errors. Both routers and bridges are capable of managing hop counts, but other types of intermediate devices (like hubs) are not.

HUB
Definition: A hub is a small, simple, inexpensive device that joins multiple computers together at a low-level network protocol layer. Most hubs manufactured today support the Ethernet standard. A person plugs one end of an Ethernet cable (that has an RJ-45 connector attached) into the hub, while the other end of the cable runs to the computer's network interface card (NIC). Non-Ethernet hubs (Token Ring, for example) also exist. One good way to differentiate between Ethernet hubs is by the speed (data rate) they support. The most basic Ethernet hubs support 10 Mbps speeds. Newer hubs support 100 Mbps Ethernet. Some hubs, to help users move ahead to the newer technology while still supporting the old, support both 10 Mbps and 100 Mbps data rates. These are dual-speed hubs. Another common differentiator in hubs is the number of ports they support. Four- and five-port hubs are most common in home networks, but eight- and 16-port hubs can be found in some home and small office environments.


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I
ICS
Definition: ICS allows a local network of Windows computers to share a single Internet connection. Microsoft developed ICS as part of Windows 98 Second Edition. The feature has been included as part of all subsequent Windows releases, but it is not available as a separate installable

program. ICS follows a client/server model. To set up ICS, one computer must be chosen as the "server." This computer must support two network interfaces, one directly connected to the Internet and the other connected to the remainder of the LAN. In a traditional home dial-up network, for example, the server computer is directly connected to the modem. When configured through Windows, the ICS server behaves as a NAT router, directing messages on behalf of multiple computers. ICS incorporates a DHCP server that allows clients to obtain their local addresses automatically rather than needing to be set manually. Compared to hardware routers, ICS has the advantage of being included with the operating system so no additional purchase is required. On the other hand, ICS lacks many of the configuration options that hardware routers possess.

Also Known As: Internet Connection Sharing

IIS
Definition: IIS is Microsoft's business-class Web server. Although Apache boasts more total worldwide installations than IIS, many corporations choose IIS as their Web server because it is supported by a commercial organization. Some have criticized IIS in the past, though, for its alleged security weaknesses. IIS has improved over the years, supports many Web standards including SSL, and integrates well with other Microsoft Web technologies such as FrontPage. An admininstrator can manage IIS through a graphical console, the Internet Services Manager.

Also Known As: Internet Information Server

INTRANET
Definition: An intranet is the generic term for the collection of private computer networks within an organization. Intranets generally use standard network technologies like Ethernet and TCP/IP. An organization's intranet often enjoys Internet access but is firewalled so that its computers cannot be reached directly from the public Net. (An extranet opens "holes" in this firewall for select outsiders.) Many schools and non-profit groups have deployed intranets, but an intranet is still seen primarily as a corporate productivity tool. In addition to email and groupware applications, intranets generally offer Web sites available internally to disseminate information. The value of an Intranet has proven difficult to quantity in terms of time or money saved.

Also Known As: private network

IP
Definition: IP is probably the world's single most popular network protocol. Data travels over an IP-based network in the form of packets; each IP packet includes both a header (that specifies source, destination, and other information about the data) and the message data itself. IP supports the notion of unique addressing for computers on a network. Current IP (IPv4) addresses contain four bytes (32 bits) that is sufficient to address most computers on the Net. IP supports protocol layering as defined in the OSI reference model. Popular higher-level protocols like HTTP, TCP, and UDP are built directly on top of IP. Likewise, IP can travel over several different lower-level data link interfaces like Ethernet and ATM. IP originated with UNIX® networking in the 1970s.

Also Known As: Internet Protocol

IP ADDRESS
Definition: An IP address is the logical address of a network adapter. The IP address uniquely identifies computers on a network. An IP address can be private, for use on a LAN, or public, for use on the Internet or other WAN. IP addresses can be determined statically (assigned to a computer by a system administrator) or dynamically (assigned by another device on the network on demand). IP addresses consist of four bytes (32 bits). Each byte of an IP address is known as an octet. Octets can take any value between 0 and 255, but various rules exist for ensuring IP addresses are valid.

Also Known As: network address

ISDN
Definition: ISDN is a network technology that supports transfer of simultaneous voice and data traffic. Similar to DSL in this respect, an ISDN Internet service works over ordinary telephone lines. ISDN Internet service generally supports data rates of 128 Kbps. ISDN emerged as an alternative to traditional dial-up networking during the 1990s. The relatively high cost of service, though, limited its popularity with home users. The much higher speeds supports by newer cable and DSL technologies diminish the importance of ISDN for home networking, but ISDN technology may still have application in other areas of business. Also Known As: Integrated Services Digital Network



ISP
Definition: An ISP is a company that provides Internet connectivity to home and business customers. ISPs choose what forms of access to provide customers, ranging from traditional modem dial-up to DSL and cable modem to T1/T3 lines. Recently, ISP companies have begun to diversify and offer additional services besides Net access. These include email, Web site and database hosting, and Web site development services and tools.

Also Known As: Internet Service Provider, Internet Access Provider (IAP)


J

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K
KBPS
Definition: One kilobit per second (Kbps) equals 1000 bits per second (bps). Kbps is also written as “kbps” that carries the same meaning. Likewise, one megabit per second (Mbps) equals one million bps and one Gigabit equals one billion bps. Network performance is best measured in bps, but sometimes numbers are given in bytes per second (Bps). Then, one KBps equals one kilobyte per second, one MBps equals one megabyte per second, and GBps equals one gigabyte per second. Many times people write KBps, for example, when they mean Kbps, and it is important to be clear on this distinction. Examples: Traditional Ethernet supports data rates up to 10 Mbps and Fast Ethernet 100 Mbps. Gigabit Ethernet supports 1000 Mbps or 1 Gbps.

Also Known As: Kb/sec, Kb/s, Mb/sec, Mb/s, Gb/sec, Gb/s

KILOBYTE
Definition: A kilobyte (KB) equals 1024 (or 210) bytes. Likewise, a megabyte (MB) equals 1024 KB or 220 bytes and a gigabyte (GB) equals 1024 MB or 230 bytes. The meaning of the words kilobyte, megabyte, and gigabyte change when they are used in the context of network data rates. A rate of one kilobyte per second (KBps) equals 1000 (not 1024) bytes per second; one megabyte per second (MBps) equals one million (106, not 220) bytes per second; one gigabyte per second (GBps) equals one billion (109, not 230) bytes per second. To avoid some of this confusion, networkers typically measure data rates in bits per second (bps) rather than bytes per second (Bps) and use the terms kilobyte, megabyte, and gigabyte only when referring to disk space. Examples: "This eight gigabyte disk only has 200 megabytes of free space left."

Also Known As: K, KB, M, MB, "meg," G, GB, "gig"


L
LAN
Definition: A LAN supplies networking capability to a group of computers in close proximity to each other such as in an office building, a school, or a home. LANs are useful for sharing files, providing access to printers and access to the Net (among other things). Most LANs are built with relatively inexpensive hardware such as Ethernet cable and network interface cards (although wireless and other options exist). Specialized operating system software is also often used to configure a LAN. For example, some flavors of Microsoft Windows -- including Windows 98 SE, Windows 2000, and Windows ME -- come with a package called Internet Connection Sharing (ICS) that support controlled access to resources on the network.

Also Known As: local area network

LEASED LINE
Definition: A leased line connects two locations for private voice or data telecommunication service. Not a dedicated cable, a leased line is really just a reserved circuit between two points that can be near to each other or separated by a long distance. Traditional telephone service reuses the same lines for many different conversations through a process of "switching," but leased lines maintain a single open circuit at all times. Leased lines most commonly are rented by businesses to connect branch offices, because these lines guarantee bandwidth for network traffic. So-called "T1" leased lines are common and offer the performance of symmetric DSL (1.544 Mbps). Individuals can also rent leased lines for high-speed Internet access, but their high cost (sometimes more than $2000 USD per month) deters most. "Fractional T1" lines, starting at 128 Kbps, reduce this cost somewhat and have been installed in some apartment buildings and hotels.

Also Known As: dedicated line


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M
MBPS
Definition: One kilobit per second (Kbps) equals 1000 bits per second (bps). Kbps is also written as “kbps” that carries the same meaning. Likewise, one megabit per second (Mbps) equals one million bps and one Gigabit equals one billion bps. Network performance is best measured in bps, but sometimes numbers are given in bytes per second (Bps). Then, one KBps equals one kilobyte per second, one MBps equals one megabyte per second, and GBps equals one gigabyte per second. Many times people write KBps, for example, when they mean Kbps, and it is important to be clear on this distinction. Examples: Traditional Ethernet supports data rates up to 10 Mbps and Fast Ethernet 100 Mbps. Gigabit Ethernet supports 1000 Mbps or 1 Gbps.

Also Known As: Kb/sec, Kb/s, Mb/sec, Mb/s, Gb/sec, Gb/s

MEGABYTE
Definition: A kilobyte (KB) equals 1024 (or 210) bytes. Likewise, a megabyte (MB) equals 1024 KB or 220 bytes and a gigabyte (GB) equals 1024 MB or 230 bytes. The meaning of the words kilobyte, megabyte, and gigabyte change when they are used in the context of network data rates. A rate of one kilobyte per second (KBps) equals 1000 (not 1024) bytes per second; one megabyte per second (MBps) equals one million (106, not 220) bytes per second; one gigabyte per second (GBps) equals one billion (109, not 230) bytes per second. To avoid some of this confusion, networkers typically measure data rates in bits per second (bps) rather than bytes per second (Bps) and use the terms kilobyte, megabyte, and gigabyte only when referring to disk space. Examples: "This eight gigabyte disk only has 200 megabytes of free space left."

Also Known As: K, KB, M, MB, "meg," G, GB, "gig"

MODEM
Definition: "Modem" is an acronym for "Modulator Demodulator." Traditionally, a modem converts data between the analog form used on telephone lines and the digital form used on computers for the purpose of computer-to-computer communication. Standard modems can transmit data at a maximum rate of 56,000 bits per second (bps) or 56kbps. However, inherent limitations of the phone system translate to modem speeds of 33.6kbps or lower in practice. More recently, modems for cable and DSL service have come to be known as digital modems and those used for traditional dial-up networking as analog modems. The terminology is somewhat misleading as modems all involve analog signaling; "digital" actually refers to improved digital support at the access provider's location and not within the modem itself. Nonetheless, cable and DSL modems use broadband signaling techniques to achieve dramatically higher network speeds than traditional modems.


N
NAS
Definition: NAS allows files to be stored and retrieved across a computer network. A NAS includes a dedicated hardware device often called the head that connects to a local area network (usually via Ethernet). This NAS "server" authenticates clients and manages file operations in much the same manner as traditional file servers, through well-established network protocols like NFS and CIFS/SMB. NAS systems attempt to reduce the cost associated with traditional file servers. Rather than utilize general-purpose computer hardware and a full-featured network operating system (NOS) like NetWare, NAS devices generally run an embedded operating system on simplified hardware. NAS boxes support hard drives, and sometimes tape drives, but lack peripherals like a monitor or keyboard. Designed specifically for network storage, a NAS tends to be easier to manage than a file server. The term "NAS" is often confused with the related term "SAN" (Storage Area Network). In a nutshell, NAS devices are just one type of entity that can exist on a SAN.

Also Known As: Network Attached Storage

NAT
Definition: NAT allows an IP-based network to manage its public (Internet) addresses separately from its private (intranet) addresses. It is a popular technology for Net connection sharing on DSL or cable LANs. With NAT, each private IP address can be translated to a different public address, or multiple private addresses can be aliased to a single public one. To accomplish this, NAT software snoops both incoming and outgoing packets on the network. It modifies the source or destination address in the IP header (and the affected checksums) to reflect the mapping between internal and external addressing for that network. NAT functionality appears on routers and other gateway devices with low-level access to packets at the network boundary. Several variations on NAT have also been implemented to provide additional support for application-level protocols. NAT for the Internet is defined in RFC 1631.

Also Known As: Network Address Translation

NETWARE
Definition: The NetWare operating system was one of the first software products built for the networking of personal computers (PCs). NetWare emphasizes file and print serving capabilities, and the predominant use of NetWare is as a LAN server. Early version of NetWare appeared in the late 1980s, and today NetWare enjoys an installed base of millions of computers. NetWare is just one of many networking software products and services developed by Novell, Inc. Despite stiff competition from Microsoft and various other companies, Novell continues to offer strong network technology.

Also Known As: Novell NetWare

NOS
Definition: A NOS is a network operating system. Network operating systems implement protocol stacks as well as device drivers for networking hardware. Some operating systems, like Windows 98 Second Edition, also add custom networking features like Internet Connection Sharing (ICS). Network operating systems have existed for more than thirty years. UNIX® was designed from the beginning to support networking. In its early forms, Windows did not support networking, so Novell NetWare became the first popular NOS for the personal computer (Windows 95 and Windows for Workgroups were Microsoft's first NOS products). Today, nearly any consumer operating system qualifies as a NOS due to the popularity of the Internet and the obvious need to support Internet Protocol (IP) at a minimum.

Also Known As: network operating system


O
OCTET
Definition: Technically, an octet represents any eight bit quantity. By definition, an octet ranges in mathematical value from zero to 255. Typically an octet is also a byte, but the term "octet" came into existence because historically some computer systems did not represent a byte as eight bits. In networking, an octet most commonly refers to any of the four bytes of an IPv4 address. In dotted-decimal notation, an IP address appears as follows - [octet] . [octet] . [octet] . [octet] for example 192 . 168 . 0. 1

Also Known As: byte

OPERATING SYSTEM
Definition: The most important program that runs on a computer. Every general-purpose computer must have an operating system to run other programs. Operating systems perform basic tasks, such as recognizing input from the keyboard, sending output to the display screen, keeping track of files and directories on the disk, and controlling peripheral devices such as disk drives and printers. For large systems, the operating system has even greater responsibilities and powers. It is like a traffic cop -- it makes sure that different programs and users running at the same time do not interfere with each other. The operating system is also responsible for security, ensuring that unauthorized users do not access the system. Operating systems can be classified as follows: • multi-user : Allows two or more users to run programs at the same time. Some operating systems permit hundreds or even thousands of concurrent users. • multiprocessing : Supports running a program on more than one CPU. • multitasking : Allows more than one program to run concurrently. • multithreading : Allows different parts of a single program to run concurrently. • real time real_time: Responds to input instantly. General-purpose operating systems, such as DOS and UNIX, are not real-time. Operating systems provide a software platform on top of which other programs, called application programs, can run. The application programs must be written to run on top of a particular operating system. Your choice of operating system, therefore, determines to a great extent the applications you can run. For PCs, the most popular operating systems are DOS, OS/2, and Windows, but others are available, such as Linux. As a user, you normally interact with the operating system through a set of commands. For example, the DOS operating system contains commands such as COPY and RENAME for copying files and changing the names of files, respectively. The commands are accepted and executed by a part of the operating system called the command processor or command line interpreter. Graphical user interfaces allow you to enter commands by pointing and clicking at objects that appear on the screen

Also Known As: The OS

OSI(MODEL)
Definition: The OSI model defines internetworking in terms of a vertical stack of layers. The upper layers of OSI represent software that implements network services like encryption and connection management. The lower layers of OSI implement more primitive functions like routing, addressing, and flow control. The OSI model was introduced in 1984. Although it was designed to be an abstract model, it remains a practical framework that many of today's key network technologies fit into.

Also Known As: Open Systems Interconnection reference model


P
P2P
Definition: P2P technically stands for "peer-to-peer." A peer-to-peer architecture allows hardware or software to function on a network without the need for central servers. Peer-to-peer is commonly used in configuring home computer networks where the cost of a server can be difficult to justify. The approach has also been popularized by some Net software applications such as Groove and Napster. The P2P acronym has acquired a non-technical meaning as well. Some people have described this second meaning as "people-to-people." From this angle, P2P is a model for developing software and growing businesses that help individuals on the Net to meet each other and share common interests.

Also Known As: Peer-to-peer

PASV
Definition: PASV refers to an alternate mode for establishing File Transfer Protocol (FTP) connections. In general, PASV mode works better when connecting through a firewall than does the normal FTP connection mode. FTP uses a two-step process to make connections between the FTP client and server. In the first step, the FTP client sends a command to the server to request connection information. In the second step, a data connection is initiated for the actual file transfer. In a nutshell, the FTP client initiates this second step when using PASV mode; otherwise, the FTP server initiates the second step when not using PASV mode. Many firewalls are configured to reject incoming communications (those initiated from a remote computer), so PASV mode is much more firewall friendly. Most FTP client programs leave PASV turned "off" by default. However, Internet Explorer and some other Web-based programs turn on PASV by default. All of these programs should allow the user to turn the PASV feature on or off through an interface option. Setting up PASV on the FTP client side does not guarantee that PASV mode will work. Most FTP servers can be configured to either allow or deny PASV mode connections. PASV entails some additional security risks on the server side, so some network administrators prefer to disable PASV mode.

Also Known As: passive mode FTP

PING
Definition: Ping is the name of a standard network utility packaged with popular network operating systems. The utility can be used to determine if a remote device (such as Web or game server) can be reached on the network and, if so, roughly how fast the current connection is. Traditional pings are implemented with the Internet Control Message Protocol (ICMP), although it is possible to achieve a similar effect with UDP or other protocols. Ping utilities send requests to the designated computer at periodic intervals and measure the time it takes for a respone packet to arrive. A number of popular "ping tools" that add features to the operating system commands are available on the Net today. By manipulating the contents of ICMP packets, hackers have also used ping utilties to take down servers in the so-called "Ping of Death" attack.

Also Known As: Packet Internet Groper, ICMP

PING OF DEATH
Definition: In late 1996 and early 1997, a flaw in the implementation of networking in some operating systems became well-known and popularized by hackers as a way to "crash" computers remotely over the Internet. The "Ping of Death" attack was relatively easy to carry out and very dangerous due to its high probability of success. Technically speaking, the attack involved sending IP packets of a size greater than 65,535 bytes to the target computer. IP packets of this size are illegal, but applications can be built that are capable of creating them. Carefully programmed operating systems could detect and safely handle illegal IP packets, but some systems failed to do this. ICMP ping utilities often included large-packet capability and became the namesake of the problem although UDP and other IP-based protocols could be used. Operating system vendors quickly devised "patches" to avoid the Ping of Death. Still, many Web sites today block ICMP ping messages at their firewalls to avoid similar denial of service attacks

PORT (NUMBER)
Definition: A port number represents an endpoint or "channel" for network communications. Port numbers allow different applications on the same computer to utilize network resources without interfering with each other. Port numbers most commonly appear in network programming, particularly socket programming. Sometimes, though, port numbers are made visible to the casual user. For example, some Web sites a person visits on the Internet use a URL like the following: http://www.mairie-metz.fr:8080/ In the above example, the number 8080 refers to the port number used by the Web browser to connect to the Web server. Normally, a Web site uses port number 80 and this number need not be included with the URL (although it can be). In IP networking, port numbers can theoretically range from 0 to 65535. Most popular network applications, though, use numbers at the low end of the range (such as 80 for HTTP). The port number is included as a field within the header of each IP packet. Note: The term "port" also refers to several other aspects of network technology. A port can refer to a physical connection point for peripheral devices such as serial, parallel, and USB ports. The term "port" is also used to refer to some Ethernet connection points, such as those on a hub, switch, or router.

Also Known As: port number, protocol number

PORTAL
Definition: A portal is a kind of Web site. The term originated with large, well-known Internet search engine sites that expanded their features to include email, news, stock quotes, and an array of other functionality. Some corporations took a similar approach in implementing their intranet sites, that then became known as enterprise information or corporate portals. Technically speaking, a portal site includes a start page with rich navigation, a collection of loosely-integrated features (some of which may be provided by partners or other third parties), and a diverse, large target audience.

Also Known As: Web site

PPPoE
Definition: PPPoE technology is being adopted by some providers of DSL service. PPPoE combines the Ethernet and Point-to-Point Protocol (PPP) standards specifically for use with modems capable of broadband connectivity. PPPoE itself is documented in RFC 2516. When used with DSL, PPPoE can require additional software that establishes a network session via dial-up. Some DSL routers, though, include built-in PPPoE support to eliminate the need for software.

Also Known As: Point to Point Protocol over Ethernet

PROTOCAL
Definition: A network protocol defines a "language" of rules and conventions for communication between devices. A protocol includes formatting rules that specify how data is packaged into messages. It also may include conventions like message acknowledgement or data compression to support reliable and/or high-performance communication. Many protocols exist in computer networking ranging from high level (like SOAP) to low level (like ARP). The Internet Protocol family includes IP and all higher-level network protocols built on top of it, such as TCP, UDP, HTTP, and FTP. Modern operating systems include services or daemons that implement support for specific protocols. Some protocols, like TCP/IP, have also been implemented in silicon hardware for optimized performance.

Also Known As: network protocol


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Q
QoS
Definition: The term QoS refers to a broad collection of networking technologies and techniques. The goal of QoS is to provide guarantees on the ability of a network to deliver predictable results. Elements of network performance within the scope of QoS often include availability (uptime), bandwidth (throughput), latency (delay), and error rate. QoS generally involves prioritization of network traffic. QoS can defined in terms of the total network "pipe," in terms of a given server's or router's performance, or in terms of specific applications. A network monitoring system must typically be deployed as part of QoS, to insure that networks are performing at the desired level. QoS is an increasingly important area of research and development in computer networking. It is especially important for the new generation of Internet applications such as video-on-demand and other consumer services. Some core Internet technologies like Ethernet were not designed to support prioritized traffic or guaranteed performance levels, making it much more difficult to implement QoS solutions. One of the most attractive features of ATM is its built-in support for QoS.

Also Known As: Quality of Service


R
RADSL
Definition: RADSL is an implementation of ADSL that automatically adjusts the connection speed to adjust for the quality of the telephone line. This feature allows RADSL service to function over longer distances than does ordinary ADSL, an important feature in suburban neighborhoods. In RADSL, the broadband modem is configured at startup to test the phone line and adjust the data rate. RADSL typically operates at a lower date rate than regular ADSL. Like ADSL, RADSL provides relatively more bandwidth for downloads and less for uploads.

Also Known As: Rate-Adaptive Asymmetric Digital Subscriber Line

REPEATER
Definition: Network repeaters regenerate incoming signals. On physical media like Ethernet, data transmissions can only span a limited distance before the quality of the signal degrades. Repeaters attempt to preserve signal integrity and extend the distance over which data can safely travel. The actual network devices that serve as repeaters are usually referred to by another name. Active hubs, for example, are sometimes called "multiport repeaters" but usually these are simply referred to as hubs. Not all hubs are repeaters, though. So-called passive hubs retransmit signals but do not regenerate them, thus they do not perform the service of a repeater. Higher-level devices in the OSI model, like switches and routers, generally do not incorporate the functions of a repeater. Technically, a repeater is a physical layer device.

RFC
Definition: RFC documents have been used on the Internet for more than 30 years. Researchers from universities and corporations publish these documents to solicit feedback on new technologies for the Internet. Most popular networking technologies like IP and Ethernet have been historically documented in RFCs. The very first RFC, RFC 1, was published in April 1969. Although the "host software" technology discussed has long since become obsolete, documents like this one offer a very interesting glimpse into the early days of computer networking. Even today, the plain text format of the RFC remains essentially the same as it has since the beginning. As the basic technologies of the Internet have matured, and the Internet has grown to include many non-technical people, the need for RFCs has diminished. Yet a few RFCs are still being produced for leading-edge research in Internet-based networking.

Also Known As: Request for Comments

ROUTER
Definition: A router is a physical device that joins multiple networks together. Technically, a router is a "layer three gateway," meaning that it connects networks (as gateways do), and that it operates at the network layer (three) of the OSI model. The home networker typically uses an Internet Protocol (IP) router, IP being the most common OSI network layer protocol. An IP router such as a DSL or cable modem router joins the home's local area network (LAN) to the wide-area network (WAN) of the Internet. By maintaining configuration information in a piece of storage called the "routing table," such routers also have the ability to filter traffic, either incoming or outgoing, based on the IP addresses of senders and receivers. Some routers allow the home networker to update the routing table from a Web browser interface. DSL and cable modem routers typically combine the functions of a router with those of a switch in a single unit.


S
SAMBA
Definition: Samba is a client/server system that implements network resource sharing for Linux and other UNIX computers. With Samba, UNIX files and printers can be shared with Windows clients and vice versa. Samba supports the Session Message Block (SMB) protocol. Nearly all Windows computers include SMB support with their internal network subsystems (NetBIOS in particular). With an appropriately-configured Samba server on Linux, Windows clients can map drives to the Linux filesystems. Likewise, the Samba client on UNIX can connect to Windows shares by their UNC name. Although differences among various operating systems (such as filesystem naming conventions, end-of-line conventions, and authentication) can limit interoperability, Samba offers a generally serviceable mechanism for resource sharing on a heterogenous network.

Also Known As: NetBIOS for UNIX

SDH
Definition: SONET is a physical layer network technology designed to carry large volumes of traffic over relatively long distances on fiber optic cabling. SONET was originally designed by ANSI for the public telephone network in the mid-1980s. SONET possesses several characteristics that make it appealing on the Internet today. It defines clear interoperability standards between different vendors' products, SONET can carry nearly any higher-level protocol (including IP), and it includes built-in support for ease of management and maintenance. SONET performs at high speeds at a cost competitive with alternatives like ATM and Gigabit Ethernet.

Also Known As: Synchronous Optical Network, Synchronous Digital Hierarchy (SDH)

SDSL
Definition: SDSL is a form of Digital Subscriber Line (DSL) service that provides equal bandwidth for both uploads and downloads. Originally developed in Europe, SDSL was one of the earliest forms of DSL to not require multiple telephone lines. SDSL possesses all of the common characteristics of DSL, including an "always on" combination of voice and data services, availability limited by physical distance, and high speed access compared to analog modems. SDSL supports data rates up to 3,088 Kbps.

Also Known As: Symmetric Digital Subscriber Line, Single-line DSL

SEGMENT
Definition: A segment is a specially-configured subset of a larger network. The boundaries of a network segment are established by devices capable of regulating the flow of packets into and out of the segment, including routers, switches, hubs, bridges, or multi-homed gateways (but not simple repeaters). Network designers create segments to physically separate related computers into groups. This grouping can improve network performance and security. In Ethernet networks, for example, computers send many broadcast packets onto the network, but only other computers on the same segment receive them. Network segments and subnets serve similar purposes; both create a grouping of computers. The difference between a segment and a subnet is as follows: a segment is a physical network construction, whereas a subnet is simply a higher-level software configuration. In particular, one cannot define a single IP subnet that functions correctly across multiple segments.

Also Known As: network segment

SHDSL
Definition: SHDSL technology can transport data symmetrically at data rates from 192 Kbps to 2,320 Kbps. SHDSL utilizes a single copper wire pair, making it an affordable DSL option attractive to small businesses.

Also Known As: Symmetric High-Bit-Rate Digital Subscriber Loop, G.shdsl

SLASHDOT EFFECT
Definition: The "Slashdot effect" is a historically interesting traffic phenomenon on the Internet. "Slashdot" refers to Slashdot.org -- a news-oriented Web site geared toward an opinionated technical audience. Slashdot attracts a loyal following that regularly visits the site's home page to track the latest headlines. The Slashdot community tends to click through on all external links posted there and, because they monitor the site so closely, viewers tend to click through very close in time to each other. These click throughs can generate a significant burst of network traffic to the external Web site, and this is the Slashdot effect. Underpowered small or personal Web sites linked to from a Slashdot.org headline can fail or be rendered unusable for a period of minutes or hours as a result of the Slashdot effect. It is effectively a denial of service (DoS) attack minus the malicious intent. Although this phenomenon attracted much attention a few years ago, sites that use a quality Web server and adequate amounts of memory (RAM) have little to fear from Slashdot.

Also Known As: /. effect

SMB
Definition: SMB is a network file sharing protocol. Communication over SMB occurs mainly through a series of client requests and server responses. SMB client and server software exists within nearly all versions of Microsoft Windows. File sharing systems using SMB, such as LAN Manager for UNIX, have also been produced for many non-Windows operating environments SMB runs at a higher level on top of other network protocols such as TCP/IP, NetBEUI, or IPX. A new version of SMB, Common Internet File System or CIFS, has been developed by Microsoft for "open" use on the Internet.

Also Known As: Server Message Block, Session Message Block, CIFS

SNIFFER
Definition: Sniffers monitor network data. A sniffer can be a self-contained software program or a hardware device with the appropriate software or firmware programming. Sniffers usually act as network probes or "snoops" -- examining network traffic but not intercepting or altering it. Some sniffers work only with TCP/IP packets, but the more sophisticated tools can work with many other protocols and at lower levels such as the Ethernet frame. In the past, sniffers were tools used exclusively by network engineers. Today, however, these utilities have become popular on the Net with hackers and the merely curious. The U.S. Federal Bureau of Investigation (FBI) has also fueled much interest in sniffer technology with the announcement of their Carnivore system.

Also Known As: network monitor

SOCKET
Definition: A socket represents a single connection between two network applications. These two applications nominally run on different computers, but sockets can also be used for interprocess communication on a single computer. Applications can create multiple sockets for communicating with each other. Sockets are bidirectional, meaning that either side of the connection is capable of both sending and receiving data. Programmers often use sockets in network programming, albeit indirectly. Programming libraries like Winsock hide many of the low-level details of socket programming. Sockets have been in widespread use since the early 1980s.

Also Known As: software object

SONET
Definition: SONET is a physical layer network technology designed to carry large volumes of traffic over relatively long distances on fiber optic cabling. SONET was originally designed by ANSI for the public telephone network in the mid-1980s. SONET possesses several characteristics that make it appealing on the Internet today. It defines clear interoperability standards between different vendors' products, SONET can carry nearly any higher-level protocol (including IP), and it includes built-in support for ease of management and maintenance. SONET performs at high speeds at a cost competitive with alternatives like ATM and Gigabit Ethernet.

Also Known As: Synchronous Optical Network, Synchronous Digital Hierarchy (SDH)

SS7
Definition: SS7 provides mechanisms for exchanging special-purpose messages on public telephone networks. Using SS7, telephone companies are able to implement modern consumer telephone services such as call forwarding. SS7 messages pass over a separate channel than that used for voice communication. This technique is commonly referred to as "out of band" signaling. Out-of-band communication is a technique commonly utilized in computer networking. The File Transfer Protocol (FTP), for example, employs a similar technique by utilizing two different channels (port numbers) for communication, one for data traffic and one for "control" information.

Also Known As: Signaling System 7

SSL
Definition: SSL security technology helps to improve the safety of Internet communications. SSL is a standard for encrypted client/server communication between network devices. A network protocol, SSL runs on top of TCP/IP. SSL utilizes several standard network security techniques including public keys, symmetric keys, and certificates. Web sites commonly use SSL to guard private information such as credit card numbers.

Also Known As: Secure Sockets Layer

SWITCH
Definition: A switch is a small device that joins multiple computers together at a low-level network protocol layer. Technically, switches operate at layer two (Data Link Layer) of the OSI model. Switches look nearly identical to hubs, but a switch generally contains more "intelligence" (and a slightly higher price tag) than a hub. Unlike hubs, switches are capable of inspecting the data packets as they are received, determining the source and destination device of that packet, and forwarding that packet appropriately. By delivering messages only to the connected device that it was intended for, switches conserve network bandwidth and offer generally better performance than hubs. Like hubs, switches primarily are available for Ethernet, come in a range of port configurations starting with the four- and five-port models, and support 10 Mbps Ethernet, 100 Mbps Ethernet, or both.

SYMMETERIC
Definition: In a symmetric network, all devices can transmit and receive data at equal rates. Asymmetric networks support more bandwidth in one direction than the other. For example, symmetric DSL offers customers identical bandwidth for both downloads and uploads. Asymmetric DSL offers more bandwidth for customer downloads at the cost of less bandwidth for uploads. A more general but less common definition of network symmetry involves access to resources. In this sense, all devices on a symmetric network share equal access to resources or the same capabilities. Asymmetric networks divide at least some resources unequally between devices. Examples: Pure peer-to-peer networks like Gnutella implement perfect symmetry: all computers on the Gnutella network have equal opportunity to discover, publish, or receive content. A hybrid peer-to-peer network like Napster is technically asymmetric because it utilizes central servers for user registration and file catalogs. However, from the point of view of clients, Napster is effectively symmetric because all customers share equivalent functionality.


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T
TCP/IP
Definition: Technically, Transmission Control Protocol (TCP) and Internet Protocol (IP) are two distinct network protocols. TCP and IP are so commonly used together, though, that "TCP/IP" has become standard terminology to refer to either *or* both of the protocols. IP roughly corresponds to the Network layer (layer 3) in the OSI model, whereas TCP corresponds to the Transport layer (layer 4) in OSI. In other words, the term TCP/IP refers to network communications where the TCP transport is used to deliver data across IP networks. The average person on the Internet works in a predominately TCP/IP environment. Web browsers, for example, use TCP/IP to communicate with Web servers.

Also Known As: Transmission Control Protocol / Internet Protocol


U
UDP
Definition: UDP is a lightweight transport built on top of IP. UDP squeezes extra performance from IP by not implementing some of the features a more heavyweight protocol (like TCP) offers. Specifically, UDP allows individual packets to be dropped (with no retries) and packets to be received in a different order than they were sent. UDP is often used in videoconferencing applications or games where optimal performance is desirable. UDP is one of the oldest network protocols, introduced in 1980 in RFC document 768.

Also Known As: User Datagram Protocol

UNC
Definition: UNC provides a naming convention for identifying network resources. UNC names consist of three parts, a server name, a share name, and an optional file path, that are combined using backslashes as follows \\server\share\file_path The server portion of a UNC path refers to names maintained by a network naming service such as DNS or WINS. Share names can be defined by a system administrator or, in some cases, exist automatically within the local operating system. For example, in Windows 2000 and earlier versions of Windows, the built-in share name admin$ refers to the root directory of the operating system installation (usually C:\WINNT or C:\WINDOWS). (Predefined share names in Windows generally end with a $, but this convention is not required for any new shares an administrator defines.) Using Windows Explorer or the DOS command prompt, a person can map to the \\computer-name\admin$ UNC share and (with proper security credentials) can remotely access the directory tree under C:\WINNT or equivalent on that computer. A person can also connect to subdirectories beneath admin$ by specifying the optional portion of the UNC name. For example, \\computer-name\admin$\system32 is the UNC name referring to C:\WINNT\system32 or equivalent on that computer. UNC notation is used primarily for mapping network drives in the Microsoft Windows family of operating systems, although support for UNC appears in related technologies like Samba. UNC names are most commonly used to reach file servers or printers on a LAN.

Also Known As: Universal Naming Convention, Uniform Naming Convention

URI
Definition: A URI is a standard global identifier for an Internet resource that may be local or remotely-accessible. URIs follow the same general syntax as URLs; in fact, URLs are one type of URI. Whereas URLs always refer to network addresses (including a protocol specification, host name or address, and local path), a URI does not necessarily refer to a remote resource. For example, the URI file:///c:/ specifies a local directory. Because file does not refer to any specific network protocol, this URI is not also a URL.

Also Known As: Uniform Resource Identifier

URL
Definition: A URL is a specially-formatted text string that defines a location on the Internet. URL strings contain three parts or substrings: 1. network protocol 2. host name or address 3. file location The network protocol substring determines the underlying Internet protocol to be used in reaching the location. These strings consist of a standard protocol name followed by the :// characters. Typical protocols found in URLs include http://, ftp://, and mailto://. The host substring immediately follows the protocol defintion. Hosts may be defined by Internet-standard naming (DNS) or by IP address. For example, a URL of of http://compnetworking.about.com or, equivalently, http://209.143.212.20 contains the protocol and host information needed to access this Web site. The file location portion of a URL defines the location of a network resource. Resources are files that can be plain text files, documents, graphics, or programs, and resource names are relative to a local root directory. Technically, a URL like http://compnetworking.about.com contains an implied file location of /, that Web servers like Apache automatically translate to a specific file name like index.htm. All other specific files exist in a hierarchy or directory tree underneath the root. COMPLETE URL http://compnetworking.about.com/library/glossary/blglossary.htm When creating HTML pages, the author can choose to use either the relative file locations or complete URLs. A user of the Internet generally works with complete URLs.

Also Known As: Uniform Resource Locator

UPSTREAM
Definition: From the user's perspective, upstream network traffic flows away from the local computer toward the remote destination. Conversely, downstream traffic flows to the user's computer. Traffic on most networks flows in both upstream and downstream directions simultaneously, and often when data flows in one direction, network protocols often send control instructions (generally invisible to the user) in the opposite direction. One way to generate upstream traffic is to upload files to a server or send an email message. Conversely, downloading files and receiving email generate downstream traffic. Typical Internet users create much more downstream than upstream traffic. Examples: The Web browser sends HTTP requests upstream to the Web server, and the server replies with downstream data usually in the form of HTML pages. Asymmetric DSL (ADSL) services provides less bandwidth in the upstream direction in order to reserve more bandwidth for downstream traffic.

USB
Definition: USB stands for Universal Serial Bus, a new standard for connecting external devices like mice, keyboards, scanners and printers. USB offers many benefits over traditional connection method, including thinner and cheaper cables, greater expandability (with the addition of a USB hub, a single USB controller can handle up to 125 devices) and greater speed. To use USB, your computer needs integrated USB ports or a USB expansion card and Windows 98; Mac users require a Mac with USB and MacOS 8.1 or greater. Pronunciation: U-S-B •

Also Known As: universal serial bus


V
VDSL
Definition: VDSL was developed to support exceptionally high-bandwidth applications such as High-Definition Television (HDTV). It is not as widely deployed as other forms of DSL service. However, VDSL can achieve data rates up to approximately 51,840 Kbps, making it the fastest available form of DSL. To perform at this speed, VDSL relies on fiber optic cabling. It is designed to work more as a business service that uses ATM internetworking rather than as a consumer service that utilizes IP. Like other forms of DSL, VDSL supports both voice and data communication on the same line. Like most DSL technologies, the performance of VDSL depends significantly on the physical distance traversed by wiring: Shorter distances mean faster networking. The technology was originally named VADSL ('A' for asymmetric), but VDSL has now been improved and can operate in either symmetric and asymmetric modes. Continue tour of the different types of DSL, or Start from the beginning of this tour

Also Known As: Very-high-speed Digital Subscriber Line, VADSL, BDSL

VORTAL
Definition: A vortal is a specialized form of portal. The term “portal” was associated first with large public search engine sites, then with corporate intranet sites. Both of these types of portals offered a breadth of functionality intended to capture as much of the available readership as possible. Later, a different form of Web site attracted the attention of analysts -- a portal geared toward a specific niche audience with focused content. The buzzword “vortal” (for “vertical portal”) was applied to this category of site to distinguish it from the others. By targeting a very focused audience, vortals in theory can provide easier-to-understand navigation, deeper content, and a place for people with common interests to meet each other.

Also Known As: vertical portal

VPN
Definition: A VPN utilizes public networks to conduct private communications. Most VPN implementations today use the Internet as the public infrastructure and a variety of specialized protocols to support private communications within the Internet. VPN follows a client and server approach. VPN clients authenticate users, encrypt data, and otherwise manage sessions with VPN servers utilizing a technique called tunneling. VPN clients and VPN servers are typically used in these three scenarios: 1. to support remote access to an intranet, 2. to support connections between multiple intranets within the same organization, and 3. to join networks between two organizations, forming an extranet. The main benefit of a VPN compared to alternatives is lower cost. Users typically interact with simple graphical VPN client programs. These applications support creating tunnels, setting configuration parameters, and connecting to and disconnecting from the VPN server. VPN solutions utilize several different network protocols including PPTP, L2TP, IPsec, and SOCKS. VPN servers can also connect directly to other VPN servers. A VPN server to server connection extends the intranet or extranet to span multiple networks. Many vendors have developed VPN hardware and software products, though some of these do not interoperate due to the immaturity of some VPN standards. Still, VPNs offer a viable alternative to traditional leased lines or remote access servers.

Also Known As: Virtual Private Networks


W
WAN
Definition: A WAN spans a large geographic area, such as a state, province or country. WANs often connect multiple smaller networks, such as LANs or MANs. The most popular WAN in the world today is the Internet. Many smaller portions of the Internet, such as extranets, are also WANs. WANs generally utilize different and much more expensive networking equipment than do LANs. Technologies sometimes found in WANs include SONET, frame relay, and ATM.

Also Known As: wide area network

WAP
Definition: WAP defines a network architecture for content delivery over wireless networks. Central to WAP's design is a network stack based on the OSI model. WAP implements several new networking protocols that perform functions similar to the well-known Web protocols HTTP, TCP, and SSL. WAP includes the concepts of browsers, servers, URLs, and gateways. WAP browsers are intended to be implemented on small mobile devices such as cell phones, pagers, and PDAs. Instead of developing content in HTML and JavaScript, WAP developers use WML and WMLScript. WAP-enabled devices exist today, although their capability is generally limited to news feeds, stock quotes, and similar basic applications. WAP is in the very early stages of development relative to other networking technologies, and its future viability remains unclear.

Also Known As: Wireless Application Protocol

WEP
Definition: WEP is a protocol that adds security to wireless local area networks based on the 802.11b standard. WEP is a data link layer (Layer 2) security technology that can be "turned on" and "turned off." It is designed to afford wireless networks the same level of protection as a comparable wired network. WEP security is based on a scheme called RC4 that involves a combination of secret user keys and system-generated values. The original implementations of WEP used so-called "40-bit" encryption, that implements a key of length 40 bits and 24 additional bits of system-generated data (64 bits total). Research has showed that 40-bit WEP contains security flaws, and consequently most product vendors today employ so-called "128-bit" encryption (key length of 104 bits, not 128 bits). When communicating over the wire, WEP keys are used to encrypt the data stream. The keys themselves do not get sent over the wire and are often stored on the wireless network adapter or in the Windows Registry. Regardless of how it is implement on a wireless LAN, WEP serves as just one element of an overall wireless LAN security strategy.

Also Known As: Wired Equivalent Privacy

WINS
Definition: The WINS supports name resolution (the automated conversion of computer names to network addresses) for Windows networks. Specifically, WINS converts NetBIOS names to IP addresses on a LAN or WAN. Like DNS, WINS employs a distributed client/server system to maintain the mapping of computer names to addresses. Windows clients can be set up through the operating system to use primary and secondary WINS servers that dynamically update name/address pairings as computers join and leave the network. The dynamic behavior of WINS means that it also supports networks using DHCP.

Also Known As: Windows Internet Naming Service

WML
Definition: WML is a tag-based language designed after the model of HTML for Web content. The designers of WML (and its companion scripting language, WMLScript) created an environment that demands less memory and processing power from browsers than HTML and JavaScript. WML also includes features that tailor it for the relatively small display sizes of today's wireless devices. WML and HTML differ in significant ways. Although WML strips some features from HTML and co-opts others, WML also incorporates some powerful programming constructs not found in HTML like variables, tasks, and events. WML implements a stricter tag syntax than HTML and includes a DTD for use with XML parsers.

Also Known As: Wireless Markup Language

WYSIWYG
Definition: What You See Is What You Get Pronunciation: whi-zee-wig • (acronym)


X
xDSL
Definition: DSL provides high-speed networking over ordinary phone lines using digital modem technology. It integrates voice and data functionality, bringing the "always on" feature of the telephone to the computer network. The technology theoretically supports data rates of 8,448 Kbps (8.448 Mbps), although typical rates vary from 1,544 Kbps to 512 Kbps and sometimes as low as 90 Kbps. DSL services target home and small business customers. However, the service only works over a limited physical distance and remains unavailable in many residential areas. The term "xDSL" generally refers to the complete family of related DSL technologies -- ADSL, SDSL, VDSL, and so on -- rather than any one specific implementation.

Also Known As: Digital Subscriber Line, Digital Subscriber Loop, xDSL

XML
Definition: eXtensible Markup Language. A markup language for writing other markup languages. XML is sometimes called a "meta" language because it describes how to write new languages. It allows for the creation of applications that are streamlined for the use of the owner. Pronunciation: ehks ehm ehl • (acronym)

Also Known As: eXtensible Markup Language Alternate Spellings: Extensible Markup Language


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