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CIDR is principally a [[bitwise]], prefix-based standard for the interpretation of IP addresses. It facilitates [[routing]] by allowing blocks of addresses to be grouped together into single [[routing table]] entries. These groups, commonly called '''CIDR blocks''', are identified using a syntax similar to that of IP addresses: a four-part dotted-decimal address, followed by a slash, then a number from 0 to 32: ''A.B.C.D/N''. The dotted-decimal portion is interpreted, like an IP address, as a 32-bit binary number that has been broken into four 8-bit bytes. The number following the slash is the '''prefix length''', the number of significant bits, counting from the left-hand side of the address. When speaking in abstract terms, the dotted-decimal portion is sometimes omitted, thus a ''/20'' is a CIDR block with an unspecified 20-bit prefix.
An IP address is part of a CIDR block, and is said to ''match'' the CIDR prefix, if the initial N bits of the address and the CIDR prefix are the same. Thus, understanding CIDR requires that IP address be visualized in [[Binary numeral system|binary]]. Since the length of an IP address is fixed at 32 bits, an N-bit CIDR prefix leaves <math>32-N</math> bits unmatched, and there are <math>2^{(32-N)}</math> possible combinations of these bits, meaning that <math>2^{(32-N)}</math> IP addresses match a given N-bit CIDR prefix. ''Shorter'' CIDR prefixes match more IP addresses, while ''longer'' CIDR prefixes match fewer. An IP address can match multiple CIDR prefixes of different lengths.
The [[Internet Assigned Numbers Authority]] (IANA) issues to [[Regional Internet Registry|Regional Internet Registries]] (RIRs) large CIDR blocks. The RIRs, each responsible for a single, large, geographic area (such as North America), then subdivide these blocks into smaller blocks and issue them to ISPs. This process can be repeated several times at different levels of delegation. Typically, an [[Internet Service Provider]] will possess one or more CIDR blocks of IP addresses.
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