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.

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