Classless Inter-domain Routing (CIDR): The Internet Address Assignment and Aggregation Plan
RFC 4632

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Network Working Group                                          V. Fuller
Request for Comments: 4632                                 Cisco Systems
BCP: 122                                                           T. Li
Obsoletes: 1519                                          Tropos Networks
Category: Best Current Practice                              August 2006

                Classless Inter-domain Routing (CIDR):
          The Internet Address Assignment and Aggregation Plan

Status of This Memo

   This document specifies an Internet Best Current Practices for the
   Internet Community, and requests discussion and suggestions for
   improvements.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2006).

Abstract

   This memo discusses the strategy for address assignment of the
   existing 32-bit IPv4 address space with a view toward conserving the
   address space and limiting the growth rate of global routing state.
   This document obsoletes the original Classless Inter-domain Routing
   (CIDR) spec in RFC 1519, with changes made both to clarify the
   concepts it introduced and, after more than twelve years, to update
   the Internet community on the results of deploying the technology
   described.

Fuller & Li              Best Current Practice                  [Page 1]
RFC 4632                 CIDR Address Strategy               August 2006

Table of Contents

   1. Introduction ....................................................3
   2. History and Problem Description .................................3
   3. Classless Addressing as a Solution ..............................4
      3.1. Basic Concept and Prefix Notation ..........................5
   4. Address Assignment and Routing Aggregation ......................8
      4.1. Aggregation Efficiency and Limitations .....................8
      4.2. Distributed Assignment of Address Space ...................10
   5. Routing Implementation Considerations ..........................11
      5.1. Rules for Route Advertisement .............................11
      5.2. How the Rules Work ........................................12
      5.3. A Note on Prefix Filter Formats ...........................13
      5.4. Responsibility for and Configuration of Aggregation .......13
      5.5. Route Propagation and Routing Protocol Considerations .....15
   6. Example of New Address Assignments and Routing .................15
      6.1. Address Delegation ........................................15
      6.2. Routing Advertisements ....................................17
   7. Domain Name Service Considerations .............................18
   8. Transition to a Long-Term Solution .............................18
   9. Analysis of CIDR's Effect on Global Routing State ..............19
   10. Conclusions and Recommendations ...............................20
   11. Status Updates to CIDR Documents ..............................21
   12. Security Considerations .......................................23
   13. Acknowledgements ..............................................24
   14. References ....................................................25
      14.1. Normative References .....................................25
      14.2. Informative References ...................................25

Fuller & Li              Best Current Practice                  [Page 2]
RFC 4632                 CIDR Address Strategy               August 2006

1.  Introduction

   This memo discusses the strategy for address assignment of the
   existing 32-bit IPv4 address space with a view toward conserving the
   address space and limiting the growth rate of global routing state.
   This document obsoletes the original CIDR spec [RFC1519], with
   changes made both to clarify the concepts it introduced and, after
   more than twelve years, to update the Internet community on the
   results of deploying the technology described.

2.  History and Problem Description

   What is now known as the Internet started as a research project in
   the 1970s to design and develop a set of protocols that could be used
   with many different network technologies to provide a seamless, end-
   to-end facility for interconnecting a diverse set of end systems.
   When it was determined how the 32-bit address space would be used,
   certain assumptions were made about the number of organizations to be
   connected, the number of end systems per organization, and total
   number of end systems on the network.  The end result was the
   establishment (see [RFC791]) of three classes of networks: Class A
   (most significant address bits '00'), with 128 possible networks each
   and 16777216 end systems (minus special bit values reserved for
   network/broadcast addresses); Class B (MSB '10'), with 16384 possible
   networks each with 65536 end systems (less reserved values); and
   Class C (MSB '110'), and 2097152 possible networks each and 254 end