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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 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.
==Assignment of CIDR blocks==
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.▼
[[Image:CIDR_Address.png|right|400px]]
▲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 publically, typically (but not always), to [[Internet Service Providers]] (ISPs). This process can be repeated several times at different levels of delegation.
In the late 1990s, the IP address 208.130.29.33 (since reassigned) was used by the <tt>www.freesoft.org</tt> web server. An analysis of this address identified three CIDR prefixes. 208.128.0.0/11, a large block containing over 2 million addresses, had been assigned by [[IANA]] to [[MCI]]. Automation Research Systems, a [[Virginia]] [[Value-added reseller|VAR]], leased an Internet connection from MCI and was assigned the 208.130.28.0/22 sub-prefix, capable of addressing just over 1000 devices. ARS used a /24 block for its public web servers, of which 208.130.29.33 was one.▼
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All of these CIDR prefixes would be used, at different locations in the network. Outside of MCI's network, the 208.128.0.0/11 prefix would be used to direct to MCI traffic bound not only for 208.130.29.33, but also for any of the roughly two million IP addresses with the same initial 11 bits. Within MCI's network, 208.130.28.0/22 would be visible, directing traffic to the leased line serving this client. Only within the Automation Research Systems corporate network would the 208.130.29.0/24 prefix be used.
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