RFC1817 - CIDR and Classful Routing

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