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Network Working Group                                         Y. RekhterRequest for Comments: 1817                                 cisco SystemsCategory: Informational                                      August 1995CIDR and Classful RoutingStatus 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.Rekhter                      Informational                      [Page 1]

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: yakov@cisco.comRekhter                      Informational                      [Page 2]