Network Working Group                                         Y. Rekhter
Request for Comments: 1817                                 cisco Systems
Category: Informational                                      August 1995


                      CIDR and Classful Routing

Status of this Memo

  This memo provides information for the Internet community.  This memo
  does not specify an Internet standard of any kind.  Distribution of
  this memo is unlimited.

Abstract

  Classless Inter-Domain Routing (CIDR) is used in the Internet as the
  primary mechanism to improve scalability of the Internet routing
  system. This document represents the IAB's (Internet Architecture
  Board) evaluation of the current and near term implications of CIDR
  on organizations that use Classful routing technology.

Background

  Classless Inter-Domain Routing (CIDR) ([RFC1518], [RFC1519]) is
  deployed in the Internet as the primary mechanism to improve scaling
  property of the Internet routing system. Essential to CIDR is the
  generalization of the concept of variable length subnet masks (VLSM)
  and the elimination of classes of network numbers (A, B, and C). The
  interior (intra-domain) routing protocols that support CIDR are OSPF,
  RIP II, Integrated IS-IS, and E-IGRP.  The exterior (inter-domain)
  routing protocol that supports CIDR is BGP-4.  Protocols like RIP,
  BGP-3, EGP, and IGRP do not support CIDR.

Implications of CIDR

  Deployment of CIDR has certain implications on the segments of the
  Internet that are still using routing technology that can not support
  CIDR. Existing sites that rely solely on a default route for their
  external connectivity may not require support of VLSM capable routing
  technology for their interior routing and CIDR for their exterior
  routing. All sites lacking support for VLSM and CIDR capable routing
  must rely on a default route, which consequently may result in a
  various degree of suboptimal routing.  Organizations that operate as
  Internet Service Providers (ISPs) are expected to be able to support
  VLSM and CIDR.






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RFC 1817               CIDR and Classful Routing             August 1995


  It is expected that in the near future the IANA will instruct the
  Internet Registries to begin allocating IP addresses out of the
  former Class A address space (64.0.0.0 through 126.0.0.0). The
  allocated blocks are going to be of variable size (based on the
  actual sites' requirements).  Sites that will use these addresses
  will have to support CIDR-capable routing protocols. All the
  providers will be required to support CIDR-capable routing protocols
  as well. Sites that do not use these addresses would be required to
  continue relying on a default route, which in turn may result in a
  various degree of suboptimal routing. If a site wants to avoid the
  suboptimality (introduced by using default route), the site will need
  to transition to CIDR-capable routing protocols.

Security Considerations

  Security issues are not discussed in this memo.

Author's Address

  Yakov Rekhter
  cisco Systems
  170 West Tasman Drive
  San Jose, CA 95134

  Phone: (914) 528-0090
  EMail: [email protected]

























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