Address Allocation for Private Internets
RFC 1918

Document Type RFC - Best Current Practice (February 1996; Errata)
Updated by RFC 6761
Obsoletes RFC 1627, RFC 1597
Also known as BCP 5
Authors Robert Moskowitz  , Daniel Karrenberg  , Yakov Rekhter  , Eliot Lear  , Geert de Groot 
Last updated 2013-03-02
Stream IETF
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Network Working Group                                         Y. Rekhter
Request for Comments: 1918                                 Cisco Systems
Obsoletes: 1627, 1597                                       B. Moskowitz
BCP: 5                                                    Chrysler Corp.
Category: Best Current Practice                            D. Karrenberg
                                                                RIPE NCC
                                                          G. J. de Groot
                                                                RIPE NCC
                                                                 E. Lear
                                                  Silicon Graphics, Inc.
                                                           February 1996

                Address Allocation for Private Internets

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.

1. Introduction

   For the purposes of this document, an enterprise is an entity
   autonomously operating a network using TCP/IP and in particular
   determining the addressing plan and address assignments within that

   This document describes address allocation for private internets. The
   allocation permits full network layer connectivity among all hosts
   inside an enterprise as well as among all public hosts of different
   enterprises. The cost of using private internet address space is the
   potentially costly effort to renumber hosts and networks between
   public and private.

2. Motivation

   With the proliferation of TCP/IP technology worldwide, including
   outside the Internet itself, an increasing number of non-connected
   enterprises use this technology and its addressing capabilities for
   sole intra-enterprise communications, without any intention to ever
   directly connect to other enterprises or the Internet itself.

   The Internet has grown beyond anyone's expectations. Sustained
   exponential growth continues to introduce new challenges.  One
   challenge is a concern within the community that globally unique
   address space will be exhausted. A separate and far more pressing
   concern is that the amount of routing overhead will grow beyond the

Rekhter, et al           Best Current Practice                  [Page 1]
RFC 1918        Address Allocation for Private Internets   February 1996

   capabilities of Internet Service Providers. Efforts are in progress
   within the community to find long term solutions to both of these
   problems. Meanwhile it is necessary to revisit address allocation
   procedures, and their impact on the Internet routing system.

   To contain growth of routing overhead, an Internet Provider obtains a
   block of address space from an address registry, and then assigns to
   its customers addresses from within that block based on each customer
   requirement. The result of this process is that routes to many
   customers will be aggregated together, and will appear to other
   providers as a single route [RFC1518], [RFC1519].  In order for route
   aggregation to be effective, Internet providers encourage customers
   joining their network to use the provider's block, and thus renumber
   their computers. Such encouragement may become a requirement in the

   With the current size of the Internet and its growth rate it is no
   longer realistic to assume that by virtue of acquiring globally
   unique IP addresses out of an Internet registry an organization that
   acquires such addresses would have Internet-wide IP connectivity once
   the organization gets connected to the Internet. To the contrary, it
   is quite likely that when the organization would connect to the
   Internet to achieve Internet-wide IP connectivity the organization
   would need to change IP addresses (renumber) all of its public hosts
   (hosts that require Internet-wide IP connectivity), regardless of
   whether the addresses used by the organization initially were
   globally unique or not.

   It has been typical to assign globally unique addresses to all hosts
   that use TCP/IP. In order to extend the life of the IPv4 address
   space, address registries are requiring more justification than ever
   before, making it harder for organizations to acquire additional
   address space [RFC1466].

   Hosts within enterprises that use IP can be partitioned into three

      Category 1: hosts that do not require access to hosts in other
                  enterprises or the Internet at large; hosts within
                  this category may use IP addresses that are
                  unambiguous within an enterprise, but may be
                  ambiguous between enterprises.

      Category 2: hosts that need access to a limited set of outside
                  services (e.g., E-mail, FTP, netnews, remote login)
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