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.


   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.


   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 ( through 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

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