C

cable modem

A device that provides Internet access over cable television lines.

cable tester

A device that is used to check for electrical continuity along a length of cable. Cable tester is a generic term that can be applied to devices such as volt/ohm meters and TDRs.

caching-only server

A type of DNS server that operates the same way as secondary servers except that a zone transfer does not take place when the caching-only server is started.

carrier

A signal that carries data. The carrier signal is modulated to create peaks and troughs, which represent binary bits.

CDDI (Copper Distributed Data Interface)

An implementation of the FDDI standard that uses copper cable rather than optical cable.

Centronics connector

A connector that uses teeth that snap into place to secure the connector.

change control

A process in which a detailed record of every change made to the network is documented.

channel

A communications path that is used for data transmission.

checksum

A basic method of error checking that involves calculating the sum of bits in a section of data and then embedding the result in the packet. When the packet reaches the destination, the calculation is performed again, to make sure the value is still the same.

CIDR (classless interdomain routing)

A technique that allows multiple addresses to be consolidated into a single entry.

circuit switching

A method of sending data between two parties, in which a dedicated circuit is created at the beginning of the conversation and broken at the end. All data transported during the session travels over the same path, or circuit.

Class A network

A TCP/IP network that uses addresses from 1 to 126 and supports up to 126 subnets with 16,777,214 unique hosts each.

Class B network

A TCP/IP network that uses addresses from 128 to 191 and supports up to 16,384 subnets with 65,534 unique hosts each.

Class C network

A TCP/IP network that uses addresses from 192 to 223 and supports up to 2,097,152 subnets with 254 unique hosts each.

client

A node that uses the services from another node on a network.

client/server networking

A networking architecture in which front-end, or client, nodes request and process data stored by the back-end, or server, node.

clustering

A technology that allows two or more computers to act as a single system to provide improved fault tolerance and load balancing.

coaxial cable

A data cable, commonly referred to as coax, that is made of a solid copper core that is insulated and surrounded by braided metal and covered with a thick plastic or rubber covering. Coax is the standard cable used in cable television and in older bus topology networks.

CONFIG

A command that is used on a NetWare server to see basic information such as the server name, NDS information, and the details of network interface configurations.

collision

The result of two frames transmitting simultaneously on an Ethernet network and colliding, thereby destroying both frames.

collision domain

A segment of an Ethernet network that is between managing nodes, where only one packet can be transmitted at any given time. Switches, bridges, and routers can be used to segment a network into separate collision domains.

collision light

An LED on networking equipment that flashes to indicate a collision on the network. a collision light can be used to determine whether the network is experiencing a large number of collisions.

COM port (communication port)

A connection through which serial devices and a computer's motherboard can communicate. A COM port requires standard configuration information, such as an IRQ, an I/O address, and a COM port number.

communication

The transfer of information between nodes on a network.

concentrator

Any device that acts as a connectivity point on a network.

connectionless communication

Packet transfer in which delivery is not guaranteed.

connection-oriented communication

Packet transfer in which delivery is guaranteed.

connectivity

The linking of nodes on a network in order for communication to take place.

copy backup

Normally, a backup of the entire hard drive. A copy backup is similar to a full backup, except the copy backup does not alter the state of the archive bits on files.

cost

A value that is used to encourage or discourage the use of a certain route through a network. Routes that are to be discouraged are assigned a higher cost, and those that are to be encouraged are assigned a lower cost.

See also [metric]

cracker

A person who attempts to break software code or gain access to a system to which he or she is not authorized.

See also [hacker]

cracking

The process of attempting to break software code, normally to defeat copyright protection or alter the software's functioning. Also the process of attempting to gain unauthorized access to a computer system.

See also [hacking]

CRC (cyclical redundancy check)

A method used to check for errors in packets that have been transferred across a network. A computation bit is added to the packet and recalculated at the destination, to determine whether the entire content of the packet has been transferred correctly.

crimper

A tool that is used to join connectors to the ends of network cables.

crossover cable

A UTP cable in which the 1 and 3 wires and the 2 and 6 wires are crossed for the purposes of placing the transmit line of one device on the receive line of the other. Crossover cables can be used to directly connect two devices—for example, two computer systems—or as a means to expand networks that use devices such as hubs or switches.

crosstalk

Electronic interference that is caused when two wires are too close to each other.

CSMA/CA (carrier-sense multiple-access with collision avoidance)

A contention media access method that uses collision-avoidance techniques.

CSMA/CD (carrier-sense multiple-access with collision detection)

A contention media access method that uses collision-detection and retransmission techniques.

CSU (channel service unit)

A network communications device that is used to connect to the digital equipment lines of the common carrier, usually over a dedicated line or Frame Relay. A CSU is used in conjunction with a DSU.

cut-through packet switching

A switching method that does not copy the entire packet into the switch buffers. Instead, the destination address is captured into the switch, the route to the destination node is determined, and the packet is quickly sent out the corresponding port. Cut-through packet switching maintains a low latency.