Classless Inter-Domain Routing
Classless Inter-Domain Routing (CIDR) is a set of administrative and technical measures, which were one of the "just in time" fixes that allowed continued growth of the public Internet which had not been planned for such growth. It dealt with the procedures for assigning Internet Protocol version 4 (IPv4) space, working around the limitations of routing technology of the time, a move to routing protocols that could usefully distribute CIDR information, and, with difficulty, a large user education problem.
Background
Internet Protocol addressing
All versions of the Internet Protocol assume that the address is split into a locator part that tells routers how to move it closer to its destination, and an identifier that brings it to the specific destination host on the final medium on the route. In the very first version of IPv4,[1]the locator field was a fixed 8 bits, allowing for a maximum of 255 possible locators (i.e., possible internconnected networks0. This fit the early research ARPANET, which principally linked large time-sharing computers; there were no personal computers, local area network, or routing within organizations.
The 8-bit locator very quickly proved inadequate, and a quick fix was applied with RFC 791. By constraining the value of the first three bits of the locator field, without changing the address length, which would need substantial equipment change, it became possible, among other things, to have more locators with different lengths, appropriate for large, medium and small networks:[2]
- Class A: 8-bit locator
- Class B: 16-bit locator
- Class C: 24-bit locator
This change also defined an address space for multicasting.
References
"Classless Routing (CIDR)", Section 4.3.2 in Computer Networks, 4th ed., Peterson & Davie (2007).
RFC-4632, BCP 122 (2006), "Classless Inter-domain Routing (CIDR): The Internet Address Assignment and Aggregation Plan", V.Fuller, T.Li, http://tools.ietf.org/html/rfc4632.