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CIDR (Classless Inter-Domain Routing, sometimes known as supernetting) is a way to allocate and specify the Internet addresses used in inter-domain routing more flexibly than with the original system of Internet Protocol (IP) address classes. As a result, the number of available Internet addresses has been greatly increased. CIDR is now the routing system used by virtually all gateway hosts on the Internet's backbone network. The Internet's regulating authorities now expect every Internet service provider (ISP) to use it for routing.
The original Internet Protocol defines IP addresses in four major classes of address structure, Classes A through D. Each of these classes 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 specified by the address. One of the most commonly used classes is (or was) Class B, which allocates space for up to 65,533 host addresses. A company who needed more than 254 host machines but far fewer than the 65,533 host addresses possible would essentially be "wasting" most of the block of addresses allocated. For this reason, the Internet was, until the arrival of CIDR, running out of address space much more quickly than necessary. CIDR effectively solved the problem by providing a new and more flexible way to specify network addresses in routers. (With a new version of the Internet Protocol - IPv6 - a 128-bit address is possible, greatly expanding the number of possible addresses on the Internet. However, it will be some time before IPv6 is in widespread use.)

Using CIDR, each IP address has a network prefix that identifies either an aggregation of network gateways or an individual gateway. The length of the network prefix is also specified as part of the IP address and varies depending on the number of bits that are needed (rather than any arbitrary class assignment structure). A destination IP address or route that describes many possible destinations has a shorter prefix and is said to be less specific. A longer prefix describes a destination gateway more specifically. Routers are required to use the most specific or longest network prefix in the routing table when forwarding packets.

CIDR is supported by the Border Gateway Protocol, the prevailing exterior (interdomain) gateway protocol. (The older exterior or interdomain gateway protocols, Exterior Gateway Protocol and Routing Information Protocol, do not support CIDR.) CIDR is also supported by the OSPF interior or intradomain gateway protocol.