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.
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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: euagate.eua.ericsson.se /pub/eua/erlang/info
means that files are available by anonymous FTP from the
host called euagate.eua.ericsson.se 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:".
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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.
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Bridge
- 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.
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Collision
- When two hosts transmit on a network at once causing
their packets to collide and corrupt each other.
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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.
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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).
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Ethernet
- 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.
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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.
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Host
- 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.
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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.
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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.
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Network
- 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.
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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.
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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.
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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).
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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.
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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.
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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.
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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.
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Protocol
- 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.
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Router
- A device which forwards packets between networks. The
forwarding decision is based on network layer information
and routing tables, often constructed by routing protocols.
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Server
- 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.
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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.
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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.
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Topology
- 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.
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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.
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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.
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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).
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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.
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Transport Layer Security protocol
- (TLS) A protocol designed to allow client/server
applications to communicate over the Internet without
eavesdropping, tampering, or message forgery.
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Tunnelling
- 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.
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Tuxedo
- 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.
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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.
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Uniform Naming Convention
- (UNC) Used in IBM PC networking to completely specify a
directory on a file server.
The basic format is:
\\servername\sharename
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.
\\server1\dave
might be set up to point to
C:\users\homedirs\dave
on a server called "server1".
It is possible to execute a program using this convention
without having to specifically link a drive, by running:
\\server\share\directory\program.exe
The undocumented DOS command, TRUENAME can be used to find
out the UNC name of a file or directory on a network drive.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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Wide Area Network
- A network, usually constructed with serial lines,
extending over distances greater than one kilometre.
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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.
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Wi-Fi
- 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.
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Windows Internet Naming Service
- (WINS) Software which resolves NetBIOS names to IP
addresses.
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Windows NT Network Model
- The network model used by Windows NT. The model has the
following layers:
User Applications (e.g. Excel)
APIs
File System Drivers
TDI
Protocols
NDIS v4
NDIS Wrapper
NDIS Card Driver
Network Adapter Card
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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.
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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.
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Wireless Application Protocol
- (WAP) An open international standard for applications
that use wireless communication, e.g. Internet access from a
mobile phone.
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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.
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Workstation
- 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, http://www.foldoc.org/,
Editor Denis Howe"
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