4.9 Glossary-Chapter 4

core router: A router that is forwarding traffic within the core of the Internet.

DHCP: Dynamic Host Configuration Protocol. DHCP is how a portable computer gets an IP address when it is moved to a new location.

edge router: A router which provides a connection between a local network and the Internet. Equivalent to “gateway”.

Host Identifier: The portion of an IP address that is used to identify a computer within a local area network.

IP Address: A globally assigned address that is assigned to a computer so that it can communicate with other computers that have IP addresses and are connected to the Internet. To simplify routing in the core of the Internet IP addresses are broken into Network Numbers and Host Identifiers. An example IP address might be “”.

NAT: Network Address Translation. This technique allows a single global IP address to be shared by many computers on a single local area network.

Network Number: The portion of an IP address that is used to identify which local network the computer is connected to.

packet vortex: An error situation where a packet gets into an infinite loop because of errors in routing tables.

RIR: Regional Internet Registry. The five RIRs roughly correspond to the continents of the world and allocate IP address for the major geographical areas of the world.

routing tables: Information maintained by each router that keeps track of which outbound link should be used for each network number.

Time To Live (TTL): A number that is stored in every packet that is reduced by one as the packet passes through each router. When the TTL reaches zero, the packet is discarded.

traceroute: A command that is available on many Linux/UNIX systems that attempts to map the path taken by a packet as it moves from its source to its destination. May be called “tracert” on Windows systems.

two-connected network: A situation where there is at least two possible paths between any pair of nodes in a network. A twoconnected network can lose any single link without losing overall connectivity.

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