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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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 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.com

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