Editing Classless Inter-Domain Routing (section)

===Assignment of CIDR blocks===
The [[Internet Assigned Numbers Authority]] (IANA) issues to [[regional Internet registries]] (RIRs) large, short-prefix CIDR blocks. However, a {{IPaddr||8}} (with over sixteen million addresses) is the largest block IANA will allocate. For example, {{IPaddr|62.0.0.0|8}} is administered by [[RIPE NCC]], the European RIR. The RIRs, each responsible for a single, large, geographic area, such as Europe or North America, subdivide these blocks and allocate subnets to local Internet registries (LIRs). Similar subdividing may be repeated several times at lower levels of delegation. End-user networks receive subnets sized according to their projected short-term need. Networks served by a single ISP are encouraged by [[IETF]] recommendations to obtain IP address space directly from their ISP. Networks served by multiple ISPs, on the other hand, may obtain [[provider-independent address space]] directly from the appropriate RIR.

[[File:CIDR Address.svg|right|400px]]
For example, in the late 1990s, the IP address {{IPaddr|208.130.29.33}} (since reassigned) was used by www.freesoft.org. An analysis of this address identified three CIDR prefixes. {{IPaddr|208.128.0.0|11}}, a large CIDR block containing over 2 million addresses, had been assigned by [[ARIN]] (the North American RIR) to [[MCI Communications|MCI]]. Automation Research Systems (ARS), a Virginia [[Value-added reseller|VAR]], leased an Internet connection from MCI and was assigned the {{IPaddr|208.130.28.0|22}} block, capable of addressing just over 1000 devices. ARS used a {{IPaddr||24}} block for its publicly accessible servers, of which {{IPaddr|208.130.29.33}} was one. All of these CIDR prefixes would be used, at different locations in the network. Outside MCI's network, the {{IPaddr|208.128.0.0|11}} prefix would be used to direct to MCI traffic bound not only for {{IPaddr|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, {{IPaddr|208.130.28.0|22}} would become visible, directing traffic to the leased line serving ARS. Only within the ARS corporate network would the {{IPaddr|208.130.29.0|24}} prefix have been used.