Classless Inter-Domain Routing: Difference between revisions

Revision as of 05:41, 10 November 2005

Revision as of 19:53, 5 December 2005

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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 IPv4 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> IPv4 addresses match a given N-bit CIDR prefix. ''Shorter'' CIDR prefixes match more addresses, while ''longer'' CIDR prefixes match fewer. An 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. A similar syntax is used: the prefix is written as an IPv6 address, followed by a slash and the number of significant bits.

==Assignment of CIDR blocks==