Site home page
(news and notices)

Get alerts when Linktionary is updated

Book updates and addendums

Get info about the Encyclopedia of Networking and Telecommunicatons, 3rd edition (2001)

Download the electronic version of the Encyclopedia of Networking, 2nd edition (1996). It's free!

Contribute to this site

Electronic licensing info



CIDR (Classless Inter-Domain Routing)

Related Entries    Web Links    New/Updated Information

Search Linktionary (powered by FreeFind)

Note: Many topics at this site are reduced versions of the text in "The Encyclopedia of Networking and Telecommunications." Search results will not be as extensive as a search of the book's CD-ROM.

In the early 1990s, Internet administrators began to address the potential problem of IP address space exhaustion. CIDR (pronounced "cider") is a solution that allows more scalability in the Internet under the current IP version 4 addressing scheme. It provides an interim solution until IP version 6 is put into place.

BGP version 4 supports the CIDR route aggregation scheme. Together, they have allowed the Internet to scale. In 1995, there were close to 65,000 routes. As CIDR aggregation has been implemented, the number of routes in the global routing table has reduced to approximately 35,000 routes. Refer to RFC 2519 (A Framework for Inter-Domain Route Aggregation, February 1999) for more information.

An important CIDR concept is "domain routing." The U.S. Postal Service ZIP code could be called a domain routing scheme. The first digit represents a large geographic area (domain). The second digit represents a region within that area (subdomain), and so on. Packages sent between domains are routed on the basis of the first digit. The remaining digits are only meaningful to the mail handlers within a domain.

This topic continues in "The Encyclopedia of Networking and Telecommunications" with a discussion of the following:

  • An analogy that helps explain how CIDR works
  • The concept of aggregation and supernetting
  • A comparison of the older Internet "class-based" addressing scheme and the CIDR scheme
  • Route flapping, black holes, and the advantages of CIDR
  • The impending exhaustion of the IPv4 32-bit address space and the advantages of IPv6
  • Service provider-based IP address allocation
  • Description of how CIDR works

In addition to this topic, you should also see "IP (Internet Protocol)" and "Routing Registries and Registries on the Internet." For an historical perspective, refer to "Routing on the Internet."

RFC 2008 (Implications of Various Address Allocation Policies for Internet Routing, October 1996) discusses how aggregation reduces the number of routes to advertise on the Internet. It also discusses address "ownership" and address "lending" concepts.

Routing protocols must support CIDR. Supporting protocols include the interior routing protocols RIP version 2 and OSPF version 2, EIGRP (Enhanced Interior Gateway Routing Protocol), and the exterior routing protocol BGP version 4. Earlier protocols like RIP, BGP-3, EGP, and IGRP do not support CIDR. Today, all ISPs and similar service providers are expected to support variable-length subnet masking and CIDR.

Despite the benefits of CIDR over the years, addressing problems still exist. This is addressed in RFC 2519 (Inter-Domain Route Aggregation, February 1999). The RFC notes that "the ability of levels of the global routing to implement efficient aggregation schemes varies widely. As a result, the size and growth rate of the Internet routing table, as well as the associated route computation required, remain major issues today. To support Internet growth, it is important to maximize the efficiency of aggregation at all levels in the routing system." It then goes on to discuss some possible solutions.

See also the related topic "NAT (Network Address Translation)." NAT runs on a gateway between two networks, usually a private network and the Internet. NAT essentially hides internal network addresses by representing all of those addresses with a single IP address to the Internet. This scheme allows internal networks to use an addressing scheme that is not officially registered. It also provides security benefits since internal addresses are hidden. Private internal IP addressing is recommended, as discussed under "IP (Internet Protocol)."

Copyright (c) 2001 Tom Sheldon and Big Sur Multimedia.
All rights reserved under Pan American and International copyright conventions.