Classless Inter-Domain Routing (CIDR): an Address Assignment and Aggregation Strategy
RFC 1519

Document Type RFC - Proposed Standard (September 1993; Errata)
Obsoleted by RFC 4632
Obsoletes RFC 1338
Last updated 2013-03-02
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Network Working Group                                          V. Fuller
Request for Comments: 1519                                       BARRNet
Obsoletes: 1338                                                    T. Li
Category: Standards Track                                          cisco
                                                                   J. Yu
                                                                   MERIT
                                                             K. Varadhan
                                                                  OARnet
                                                          September 1993

                 Classless Inter-Domain Routing (CIDR):
             an Address Assignment and Aggregation Strategy

Status of this Memo

   This RFC specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" for the standardization state and status
   of this protocol.  Distribution of this memo is unlimited.

Abstract

   This memo discusses strategies for address assignment of the existing
   IP address space with a view to conserve the address space and stem
   the explosive growth of routing tables in default-route-free routers.

Table of Contents

   Acknowledgements .................................................  2
   1.  Problem, Goal, and Motivation ................................  2
   2.  CIDR address allocation ......................................  3
   2.1  Aggregation and its limitations .............................  3
   2.2  Distributed network number allocation .......................  5
   3.  Cost-benefit analysis ........................................  6
   3.1  Present allocation figures ..................................  7
   3.2  Historic growth rates .......................................  8
   3.3  Detailed analysis ...........................................  8
   3.3.1  Benefits of new addressing plan ...........................  9
   3.3.2  Growth rate projections ...................................  9
   4.  Changes to inter-domain routing protocols and practices ...... 11
   4.1  Protocol-independent semantic changes ....................... 11
   4.2  Rules for route advertisement ............................... 11
   4.3  How the rules work .......................................... 13
   4.4  Responsibility for and configuration of aggregation ......... 14
   4.5  Intra-domain protocol considerations ........................ 15
   5.  Example of new allocation and routing ........................ 15

Fuller, Li, Yu & Varadhan                                       [Page 1]
RFC 1519                 CIDR Address Strategy            September 1993

   5.1  Address allocation .......................................... 15
   5.2  Routing advertisements ...................................... 17
   6.  Extending CIDR to class A addresses .......................... 18
   7.  Domain Naming Service considerations ......................... 20
   7.1 Procedural changes for class-C "supernets" ................... 20
   7.2 Procedural changes for class-A subnetting .................... 21
   8.  Transitioning to a long term solution ........................ 22
   9.  Conclusions .................................................. 22
   10.  Recommendations ............................................. 22
   11.  References .................................................. 23
   12.  Security Considerations ..................................... 23
   13.  Authors' Addresses .......................................... 24

Acknowledgements

   The authors wish to express their appreciation to the members of the
   ROAD group with whom many of the ideas contained in this document
   were inspired and developed.

1.  Problem, Goal, and Motivation

   As the Internet has evolved and grown over in recent years, it has
   become evident that it is soon to face several serious scaling
   problems. These include:

      1.   Exhaustion of the class B network address space. One
           fundamental cause of this problem is the lack of a network
           class of a size which is appropriate for mid-sized
           organization; class C, with a maximum of 254 host
           addresses, is too small, while class B, which allows up to
           65534 addresses, is too large for most organizations.

      2.   Growth of routing tables in Internet routers beyond the
           ability of current software, hardware, and people to
           effectively manage.

      3.   Eventual exhaustion of the 32-bit IP address space.

   It has become clear that the first two of these problems are likely
   to become critical within the next one to three years.  This memo
   attempts to deal with these problems by proposing a mechanism to slow
   the growth of the routing table and the need for allocating new IP
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