Classless Inter-Domain Routing: Difference between revisions

Revision as of 05:55, 9 November 2005

Revision as of 06:18, 9 November 2005

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[[Image:CIDR_Address.png|right|400px]]

The [[Internet Assigned Numbers Authority]] (IANA) issues to [[Regional Internet Registry|Regional Internet Registries]] (RIRs) large, short-prefix CIDR blocks. For example, 62.0.0.0/8, with over sixteen million addresses, is administered by [[RIPE]], the European RIR. The RIRs, each responsible for a single, large, geographic area (such as Europe or North America), then subdivide these blocks into smaller blocks and issue them publically,. typically (butThis notsubdividing always),process tocan be repeated several times at different levels of delegation. Large [[Internet Service Providers]] (ISPs). typically Thisobtain processCIDR canblocks befrom repeateda severalRIR, timesthen atsubdivide differentthem levelsinto smaller CIDR blocks for their subscribers, sized according to the size of delegationthe subscriber's network. Networks served by a single ISP are encouraged by [[IETF]] to obtain IP address space directly from their ISP. Networks served by multiple ISPs, on the other hand, will often issueobtain independent CIDR blocks todirectly subscribersfrom the appropriate RIR.

For example, 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 [[ARIN]] (the North American RIR) 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.