Computer Networking Glossary

Access Control List

(ACL) A list of the services available on a server, each with a list of the hosts permitted to use the service.

Anonymous FTP

An interactive service provided by many Internet hosts allowing any user to transfer documents, files, programs, and other archived data using File Transfer Protocol. The user logs in using the special user name "ftp" or "anonymous" and his e-mail address as password. He then has access to a special directory hierarchy containing the publically accessible files, typically in a subdirectory called "pub". This is usually a separate area from files used by local users.

A reference like

ftp: /pub/eua/erlang/info

means that files are available by anonymous FTP from the host called in the directory (or file) /pub/eua/erlang/info. Sometimes the hostname will be followed by an Internet address in parentheses. The directory will usually be given as a path relative to the anonymous FTP login directory. A reference to a file available by FTP may also be in the form of a URL starting "ftp:".

Application Layer

The top layer of the OSI seven layer model. This layer handles issues like network transparency, resource allocation and problem partitioning. The application layer is concerned with the user's view of the network (e.g. formatting electronic mail messages). The presentation layer provides the application layer with a familiar local representation of data independent of the format used on the network.


A device which forwards traffic between network segments based on data link layer information. These segments would have a common network layer address.

Every network should only have one root bridge.


When two hosts transmit on a network at once causing their packets to collide and corrupt each other.

Collision Detection

A class of methods for sharing a data transmission medium in which hosts transmit as soon as they have data to send and then check to see whether their transmission has suffered a collision with another host's.

If a collision is detected then the data must be resent. The resending algorithm should try to minimise the chance that two hosts's data will repeatedly collide. For example, the CSMA/CD protocol used on Ethernet specifies that they should then wait for a random time before re-transmitting.

Data Link Layer

Layer two, the second lowest layer in the OSI seven layer model. The data link layer splits data into frames for sending on the physical layer and receives acknowledgement frames. It performs error checking and re-transmits frames not received correctly. It provides an error-free virtual channel to the network layer. The data link layer is split into an upper sublayer, Logical Link Control (LLC), and a lower sublayer, Media Access Control (MAC).


A local area network first described by Metcalfe & Boggs of Xerox PARC in 1976. Specified by DEC, Intel and XEROX (DIX) as IEEE 802.3 and now recognised as the industry standard.

Data is broken into packets and each one is transmitted using the CSMA/CD algorithm until it arrives at the destination without colliding with any other packet. The first contention slot after a transmission is reserved for an acknowledge packet. A node is either transmitting or receiving at any instant. The bandwidth is about 10 Mbit/s. Disk-Ethernet-Disk transfer rate with TCP/IP is typically 30 kilobyte per second.

Version 2 specifies that collision detect of the transceiver must be activated during the inter-packet gap and that when transmission finishes, the differential transmit lines are driven to 0V (half step). It also specifies some network management functions such as reporting collisions, retries and deferrals.

Ethernet cables are classified as "XbaseY", e.g. 10base5, where X is the data rate in Mbps, "base" means "baseband" (as opposed to radio frequency) and Y is the category of cabling. The original cable was 10base5 ("full spec"), others are 10base2 ("thinnet") and 10baseT ("twisted pair") which is now (1998) very common. 100baseT ("Fast Ethernet") is also increasingly common.

File Transfer Protocol

(FTP) A client-server protocol which allows a user on one computer to transfer files to and from another computer over a TCP/IP network. Also the client program the user executes to transfer files. It is defined in STD 9, RFC 959.


A computer connected to a network.

The term node includes devices such as routers and printers which would not normally be called "hosts".

Because the network is known to cover only a small area, optimisations can be made in the network signal protocols that permit data rates up to 100Mb/s.

Local Area Network

(LAN) A data communications network which is geographically limited (typically to a 1 km radius) allowing easy interconnection of terminals, microprocessors and computers within adjacent buildings. Ethernet and FDDI are examples of standard LANs.

Because the network is known to cover only a small area, optimisations can be made in the network signal protocols that permit data rates up to 100Mb/s.

Metropolitan Area Network

(MAN) A data network intended to serve an area the size of a large city. Such networks are being implemented by innovative techniques, such as running optical fibre through subway tunnels. A popular example of a MAN is SMDS.


Hardware and software data communication systems.

The OSI seven layer model attempts to provide a way of partitioning any computer network into independent modules from the lowest (physical) layer to the highest (application) layer. Many different specifications exist at each of these layers.

Networks are often also classified according to their geographical extent: local area network (LAN), metropolitan area network (MAN), wide area network (WAN) and also according to the protocols used.

Network Address

The network portion of an IP address. For a class A network, the network address is the first byte of the IP address. For a class B network, the network address is the first two bytes of the IP address. For a class C network, the network address is the first three bytes of the IP address. In each case, the remainder is the host address. In the Internet, assigned network addresses are globally unique.

Network Layer

(communications subnet layer) The third lowest layer in the OSI seven layer model. The network layer determines routing of packets of data from sender to receiver via the data link layer and is used by the transport layer. The most common network layer protocol is IP.

Network Management

The process of controlling a network so as to maximise its efficiency and productivity. ISO's model divides network management into five categories: fault management, accounting management, configuration management, security management and performance management.

Fault management is the process of identifying and locating faults in the network. This could include discovering the existence of the problem, identifying the source, and possibly repairing (or at least isolating the rest of the network from) the problem.

Configuration management is the process of identifying, tracking and modifying the setup of devices on the network. This category is extremely important for devices that come with numerous custom settings (e.g. routers and file servers).

Security management is the process of controlling (granting, limiting, restricting or denying) access to the network and resources thereon. This could include setting up and managing access lists in routers (creating "firewalls" to keep intruders out), creating and maintaining password access to critical network resources, identifying the points of entry used by intruders and closing them.

Performance Management is the process of measuring the performance of various network components. This also includes taking measures to optimise the network for maximum system performance (periodically measuring of the use of network resources).

Network Transparency.

A feature of an operating system or other service which lets the user access a remote resource through a network without having to know if the resource is remote or local.

Open Systems Interconnection

(OSI-RM, OSI Reference Model, seven layer model) A model of network architecture and a suite of protocols (a protocol stack) to implement it, developed by ISO in 1978 as a framework for international standards in heterogeneous computer network architecture.

The OSI architecture is split between seven layers, from lowest to highest: 1 physical layer, 2 data link layer, 3 network layer, 4 transport layer, 5 session layer, 6 presentation layer, 7 application layer.

Each layer uses the layer immediately below it and provides a service to the layer above. In some implementations a layer may itself be composed of sub-layers.

Physical Layer

Layer one, the lowest layer in the OSI seven layer model. The physical layer encompasses details such as electrical and mechanical connections to the network, transmission of binary data as changing voltage levels on wires or similar concepts on other connectors, and data rates.

The physical layer is used by the data link layer.

Presentation Layer

The second highest layer (layer 6) in the OSI seven layer model. Performs functions such as text compression, code or format conversion to try to smooth out differences between hosts. Allows incompatible processes in the application layer to communicate via the session layer.

The physical layer is used by the data link layer.


A set of formal rules describing how to transmit data, especially across a network. Low level protocols define the electrical and physical standards to be observed, bit- and byte-ordering and the transmission and error detection and correction of the bit stream. High level protocols deal with the data formatting, including the syntax of messages, the terminal to computer dialogue, character sets, sequencing of messages etc.

The physical layer is used by the data link layer.


A device which forwards packets between networks. The forwarding decision is based on network layer information and routing tables, often constructed by routing protocols.


A computer which provides some service for other computers connected to it via a network. The most common example is a file server which has a local disk and services requests from remote clients to read and write files on that disk, often using Sun's Network File System (NFS) protocol or Novell Netware on PCs. Another common example is a web server.

Session Layer

The third highest protocol layer (layer 5) in the OSI seven layer model. The session layer uses the transport layer to establish a connection between processes on different hosts. It handles security and creation of the session. It is used by the presentation layer.

Token Bus

(IEEE 802.4) A networking protocol which mediates access to a bus topology network as though it were a token ring. This eliminates the collisions found in carrier sense collision detect protocols. Nodes can be configured to pass the token in any order, not necessarily related to their physical ordering on the bus. The token is sent from one node to its successor in the logical ring by broadcast on the bus and is ignored by the other nodes.


Which hosts are directly connected to which other hosts in a network. Network layer processes need to consider the current network topology to be able to route packets to their final destination reliably and efficiently.

Transit Network

A network which passes traffic between other networks in addition to carrying traffic for its own hosts. It must have paths to at least two other networks.

Transmission Control Protocol

(TCP) The most common transport layer protocol used on Ethernet and the Internet. It was developed by DARPA.

TCP is the connection-oriented protocol built on top of Internet Protocol (IP) and is nearly always seen in the combination TCP/IP (TCP over IP). It adds reliable communication and flow-control and provides full-duplex, process-to-process connections.

TCP is defined in STD 7 and RFC 793.

User Datagram Protocol is the other, connectionless, protocol that runs on top of IP.

Transport Layer

(Or "host-host layer") The middle layer in the OSI seven layer model. The transport layer determines how to use the network layer to provide a virtual error-free, point to point connection so that host A can send messages to host B and they will arrive un-corrupted and in the correct order. It establishes and dissolves connections between hosts. It is used by the session layer.

An example transport layer protocol is Transmission Control Protocol (TCP).

Transport Layer Interface

(TLI, or "Transport Level Interface") A protocol-independent interface for accessing network facilities, modelled after the ISO transport layer (level 4), that first appeared in Unix SVR3.

TLI is defined by SVID as transport mechanism for networking interfaces, in preference to sockets, which are biased toward IP and friends. A disavantage is that a process cannot use read/write directly, but has to use backends using stdin and stdout to communicate with the network connection. TLI is implemented in SVR4 using the STREAMS interface. It adds no new system calls, just a library, libnsl_s.a. The major functions are t_open, t_bind, t_connect, t_listen, t_accept, t_snd, t_rcv, read, write.

According to the Solaris t_open man page, XTI (X/OPEN Transport Interface) evolved from TLI, and supports the TLI API for compatibility, with some variations on semantics.

Transport Layer Security protocol

(TLS) A protocol designed to allow client/server applications to communicate over the Internet without eavesdropping, tampering, or message forgery.


Encapsulation of protocol A within protocol B, such that A treats B as though it were a data link layer. Tunnelling is used to get data between administrative domains which use a protocol that is not supported by the internet connecting those domains.


Cross-platform distributed transaction monitor middleware marketed by BEA systems. Tuxedo supports the production of scalable client-server applications and the coordination of transactions spanning heterogeneous databases, operating systems, and hardware.

Twisted Pair

A type of networking cable in which pairs of conductors are twisted together to randomise possible cross-talk from nearby wiring. Inadequate twisting is detectable using modern cable testing instruments.

Uniform Naming Convention

(UNC) Used in IBM PC networking to completely specify a directory on a file server.

The basic format is:


where "servername" is the hostname of a network file server, and "sharename" is the name of a networked or shared directory. Note this is not the same as the conventional MS-DOS "C:\windows" directory name. E.g.


might be set up to point to


on a server called "server1".

It is possible to execute a program using this convention without having to specifically link a drive, by running:


The undocumented DOS command, TRUENAME can be used to find out the UNC name of a file or directory on a network drive.

Value Added Network

(VAN) A privately owned network that provides a specific service, such as legal research or access to a specialised database, for a fee. A Value Added Network usually offers some service or information that is not readily available on public networks.

A Value Added Network's customers typically purchase leased lines that connect them to the network or they use a dial-up number, given by the network owner, to gain access to the network.

Vampire Tap

A device to connect a network node to an RG8 thick ethernet cable without affecting other connected nodes.

A vampire tap has an interface box with a "V" shaped groove along one side. A sharp needle protrudes from the center of the groove. The cable is clamped into the groove by a grooved plate held in position by two thumb screws. With sufficient practise, tightening the screws forces the needle through the cable jacket and into contact with the cable's center wire while other spikes bite into the outer conductor. The interface box has a 15 pin connector to connect to the network node.

The vampire tap is often built into the transceiver, with a more flexible multi-wire "drop cable" to connect the transceiver to the node.

Virtual Host

Most computers on the Internet have a single Internet address; however, often via special kernel patches, a given computer can be made to respond to several IP addresses and provide different services (typically different Web services) on each. Each of these different IP addresess (which generally each have their own hostname) act as if they were distinct hosts on distinct machines, even though they are actually all one host. Hence, they are virtual hosts. A common use is when an Internet Service Provider "hosts" World-Wide Web or other services for several of their customers on one computer but giving the appearence that they are separate servers.

Virtual LAN

Software defined groups of host on a local area network (LAN) that communicate as if they were on the same wire, even though they are physically on different LAN segments throughout a site. To define a virtual LAN, the network administrator uses a virtual LAN management utility to establish membersip rules that determine which hostss are in a specific virtual LAN.

Virtual Loadable Module

(VLM) Novell's term for software modules that can be dynamically loaded to extend the functionality of the "VLM" NetWare Requester for MS-DOS that became standard beginning with Novell NetWare 4.

Virtual Local Area Network

(VLAN) A logical grouping of two or more nodes which are not necessarily on the same physical network segment but which share the same IP network number. This is often associated with switched Ethernet.

Virtual Path

The location of a file or directory on a particular server, as seen by a remote client accessing it via World-Wide Web (or similar distributed document service).

A virtual path provides access to files outside the default directory and subdirectories. It appears in the form ".../~name/..." where "~name" is replaced with actual path configured by the administrator. An access control list can be associated with a virtual path.

Virtual Private Network

(VPN) The use of encryption in the lower protocol layers to provide a secure connection through an otherwise insecure network, typically the Internet. VPNs are generally cheaper than real private networks using private lines but rely on having the same encryption system at both ends. The encryption may be performed by firewall software or possibly by routers.

Well-known Port

A TCP or UDP port with a number in the range 0-1023 (originally 0-255). The well-known port numbers are assigned by the IANA and on most systems can only be used by system (or root) processes or by programs executed by privileged users.

Wide Area Information Servers

(WAIS) A distributed information retrieval system. WAIS is supported by Apple Computer, Thinking Machines and Dow Jones. Clients are able to retrieve documents using keywords. The search returns a list of documents, ranked according to the frequency of occurrence of the keyword(s) used in the search. The client can retrieve text or multimedia documents stored on the server. WAIS offers simple natural language input, indexed searching for fast retrieval, and a "relevance feedback" mechanism which allows the results of initial searches to influence future searches. It uses the ANSI Z39.50 service. Public domain implementations are available.

Wide Area Network

A network, usually constructed with serial lines, extending over distances greater than one kilometre.

Wideband ATM

An enhanced form of ATM networking that transfers digital data over local area networks, originally at 0.96 Gbps, now (Aug 1996) at 1.0 Gbps.


Either of two different incompatible radio-based LAN protocols, namely 802.11b (which speaks DSSS at 2.4GHz) and 802.11a (which speaks OFDM at 5GHz).

The term was invented by the marketing departments of wi-fi equipment manufacturers. It is, notionally, short for "wireless fidelity", on the analogy of hi-fi for "high fidelity" audio.

Windows Internet Naming Service

(WINS) Software which resolves NetBIOS names to IP addresses.

Windows NT Network Model

The network model used by Windows NT. The model has the following layers:

User Applications (e.g. Excel)
File System Drivers
NDIS Wrapper
NDIS Card Driver
Network Adapter Card

Windows Sockets

(Winsock) A specification for Microsoft Windows network software, describing how applications can access network services, especially TCP/IP. Winsock is intended to provide a single API to which application developers should program and to which multiple network software vendors should conform. For any particular version of Microsoft Windows, it defines a binary interface (ABI) such that an application written to the Windows Sockets API can work with a conformant protocol implementation from any network software vendor.

Wireless Networking

A term describing a computer network where there is no physical connection (either copper cable or fibre optics) between sender and receiver, but instead they are connected by radio.

Wireless Application Protocol

(WAP) An open international standard for applications that use wireless communication, e.g. Internet access from a mobile phone.

Wireless Local Area Network

(WLAN /W-lan/, or "LAWN" /lorn/, sometimes "WiLAN" /wi-lan/) A communication system that transmits and receives data using modulated electromagnetic waves, implemented as an extension to, or as an alternative for, a wired LAN. WLANs are typically found within a small client node-dense locale (e.g. a campus or office building), or anywhere a traditional network cannot be deployed for logistical reasons.

Benefits include user mobility in the coverage area, speed and simplicity of physical setup, and scalability. Being a military spin-off, WLANs also provide security features such as encryption, frequency hopping, and firewalls. Some of these are intrinsic to the protocol, making WLANs at least as secure as wired networks, and usually more so. The drawbacks are high initial cost (mostly hardware), limited range, possibility of mutual interference, amd the need to security-enable clients.


A general-purpose computer designed to be used by one person at a time and which offers higher performance than normally found in a personal computer, especially with respect to graphics, processing power and the ability to carry out several tasks at the same time.

Source: "The Free On-line Dictionary of Computing,, Editor Denis Howe"