supernetting or Classless Inter-Domain Routing (CIDR)

Contributor(s): R. Damian Koziel

Also see subnet.

Supernetting, also called Classless Inter-Domain Routing (CIDR), is a way to aggregate multiple Internet addresses of the same class. The original Internet Protocol (IP) defines IP addresses in four major classes of address structure, Classes A through D. Each class allocates one portion of the 32-bit Internet address format to a network address and the remaining portion to the specific host machines within the network. Using supernetting, the network address and an adjacent address can be merged into The "23" at the end of the address says that the first 23 bits are the network part of the address, leaving the remaining nine bits for specific host addresses. Supernetting is most often used to combine Class C network addresses and is the basis for most routing protocols currently used on the Internet.

Supernetting was created as a way to solve the problem of routing tables growing beyond the ability of current software and people to manage and to provide a solution to the exhaustion of Class B network address space. Supernetting allows one routing table entry to represent an aggregation of networks much like one area code represents an aggregation of telephone numbers in an area.

The Border Gateway Protocol (BGP), the prevailing exterior (interdomain) gateway protocol and the Open Shortest Path First (OSPF) router protocol both support supernetting, but the older exterior or interdomain protocols, the Exterior Gateway Protocol (EGP) and the Routing Information Protocol (RIP) do not support it.

This was last updated in April 2007

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