Network Working Group                                        Y. Rekhter
Request for Comments: 1597       T.J. Watson Research Center, IBM Corp.
Category: Informational                                    B. Moskowitz
                                                         Chrysler Corp.
                                                          D. Karrenberg
                                                               RIPE NCC
                                                            G. de Groot
                                                               RIPE NCC
                                                             March 1994


                Address Allocation for Private Internets

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.

1. Introduction

   This RFC describes methods to preserve IP address space by not
   allocating globally unique IP addresses to hosts private to an
   enterprise while still permitting full network layer connectivity
   between all hosts inside an enterprise as well as between all public
   hosts of different enterprises. The authors hope, that using these
   methods, significant savings can be made on allocating IP address
   space.

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

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 current practice is to assign globally unique addresses to all
   hosts that use TCP/IP.  There is a growing concern that the finite IP
   address space might become exhausted.  Therefore, the guidelines for
   assigning IP address space have been tightened in recent years [1].
   These rules are often more conservative than enterprises would like,
   in order to implement and operate their networks.




 
RFC 1597        Address Allocation for Private Internets      March 1994


   Hosts within enterprises that use IP can be partitioned into three
   categories:

      - hosts that do not require access to hosts in other enterprises
        or the Internet at large;

      - hosts that need access to a limited set of outside services
        (e.g., E-mail, FTP, netnews, remote login) which can be handled
        by application layer gateways;

      - hosts that need network layer access outside the enterprise
        (provided via IP connectivity);

      - hosts within the first category may use IP addresses that are
        unambiguous within an enterprise, but may be ambiguous between
        enterprises.

   For many hosts in the second category an unrestricted external access
   (provided via IP connectivity) may be unnecessary and even
   undesirable for privacy/security reasons.  Just like hosts within the
   first category, such hosts may use IP addresses that are unambiguous
   within an enterprise, but may be ambiguous between enterprises.

   Only hosts in the last category require IP addresses that are
   globally unambiguous.

   Many applications require connectivity only within one enterprise and
   do not even need external connectivity for the majority of internal
   hosts.  In larger enterprises it is often easy to identify a
   substantial number of hosts using TCP/IP that do not need network
   layer connectivity outside the enterprise.

   Some examples, where external connectivity might not be required,
   are:

      - A large airport which has its arrival/departure displays
        individually addressable via TCP/IP. It is very unlikely that
        these displays need to be directly accessible from other