CIDR vs Classful: CIDR and Classful Routing
How CIDR affected networks using older address technology
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
AI ELI5
On this page
§1The CIDR Transition
CIDR (Classless Inter-Domain Routing) fundamentally changed how the Internet assigns and routes IP addresses. In the old classful system, addresses came in three fixed-size classes: Class A networks (16 million addresses each), Class B networks (65,536 addresses each), and Class C networks (256 addresses each). An organization got one whole class, no matter if it needed only a fraction of that space.
CIDR eliminated classes and instead allows addresses to be allocated in blocks of any size. A small organization can get exactly 512 addresses; a large one can get 65,536. This efficient allocation, combined with route aggregation (grouping multiple networks into one routing table entry), reduced the explosive growth of routing tables that was threatening the Internet's scalability.
The old way: you get a whole huge block of addresses whether you needed them or not. CIDR: you get only what you actually need, and routers can group them together in the routing table.
§2CIDR-Capable vs. Classful Routing
To use CIDR effectively, interior routing protocols (those used within an organization) must support Variable Length Subnet Masks (VLSM). The protocols that support CIDR are OSPF, RIP II, Integrated IS-IS, and EIGRP. The exterior protocol that supports CIDR is BGP-4 (Border Gateway Protocol version 4).
Older protocols do not support CIDR. RIP (the original version), BGP-3, EGP, and IGRP cannot understand the variable-size address blocks that CIDR uses. These protocols were designed when classes existed, and they still assume class-based addressing.
An organization operating as an Internet Service Provider (ISP) must support CIDR-capable routing to participate fully in the Internet. However, smaller sites that rely on a single default route for external connectivity may not strictly require CIDR support internally, though they lose some routing efficiency.
New protocols understand CIDR and can be smart about routing. Old protocols do not understand it. If you have old equipment, you have to fall back to a default route and accept worse performance.
§3Implications for Organizations
The IANA (Internet Assigned Numbers Authority) was beginning to allocate IP addresses from what had been the Class A address space (64.0.0.0 through 126.0.0.0) in variable-sized blocks based on actual organizational requirements. Sites receiving these addresses were required to support CIDR-capable routing.
For organizations using old classful routing technology, there were two choices: upgrade to CIDR-capable routing protocols, or continue using default routes and accept suboptimal routing. A default route sends all traffic destined for unknown networks to a single gateway, which may result in inefficient paths compared to the optimal route. Organizations hoping to avoid this suboptimality would need to transition to CIDR-capable protocols like OSPF or BGP-4.
The memo effectively served as notice that the Internet's architecture was changing, and organizations needed to prepare for the transition or accept degraded performance.
If you want good routing, you have to upgrade your equipment to understand CIDR. If you do not, your traffic will take longer paths, but you can still use the Internet.