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[[Image:IP_Address_Match.png|400px|right]]
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''', share an initial sequence of bits in the [[Binary numeral system|binary]] representation of their addresses, and 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
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.
CIDR is also be used with IPv6 addresses, where the prefix length can range from 0 to 128, due to the larger number of bits in the address.
==Assignment of CIDR blocks==
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