CIDR vs Classful: CIDR and Classful Routing
Describes how CIDR affects organizations that still use classful routing protocols.
Rekhter, Y., "CIDR and Classful Routing", RFC 1817, DOI 10.17487/RFC1817, August 1995, <https://www.rfc-editor.org/info/rfc1817>.
@misc{rfc1817,
series = {Request for Comments},
number = 1817,
howpublished = {RFC 1817},
publisher = {RFC Editor},
doi = {10.17487/RFC1817},
url = {https://www.rfc-editor.org/info/rfc1817},
author = {Y. Rekhter},
title = {{CIDR and Classful Routing}},
pagetotal = 2,
year = 1995,
month = aug,
} RFC text
Contents
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
EMail: yakov@cisco.com
ISO Plain Language
On this page
§1What CIDR Changes
CIDR uses variable-length subnet masks and removes the old classes of network numbers A, B, and C. The memo identifies OSPF, RIP II, Integrated IS-IS, E-IGRP, and BGP-4 as protocols that support CIDR in the roles described.
It identifies RIP, BGP-3, EGP, and IGRP as protocols that do not support CIDR.
CIDR allows address blocks to be described without the old fixed network classes. Routers need protocols that can carry this classless routing information.
§2Sites Using Classful Routing
A site that relies only on a default route for external connectivity may not need CIDR support in its internal routing protocol. Other sites that lack variable-length subnet mask and CIDR support must rely on a default route.
The memo warns that this can lead to different degrees of suboptimal routing. An organization that wants to avoid this effect needs to move to CIDR-capable routing protocols.
A site without CIDR-capable routing may send external traffic through a default route. The resulting path may be less direct than one chosen with more specific route information.
§3Address Allocation and Providers
The memo anticipates that Internet registries will allocate blocks of variable size from former Class A address space based on the needs of sites. Sites using these blocks will need CIDR-capable routing.
It says providers are expected to support CIDR-capable routing protocols, while sites not using the new addresses could continue to rely on default routes.
Variable-size address allocations require routing protocols that understand CIDR. Providers are expected to support those protocols so the routes can be carried through the Internet.