The IP Network Address Translator (NAT)
RFC 1631

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Network Working Group                                         K. Egevang
Request for Comments: 1631                           Cray Communications
Category: Informational                                       P. Francis
                                                                     NTT
                                                                May 1994

                The IP Network Address Translator (NAT)

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

   The two most compelling problems facing the IP Internet are IP
   address depletion and scaling in routing. Long-term and short-term
   solutions to these problems are being developed. The short-term
   solution is CIDR (Classless InterDomain Routing). The long-term
   solutions consist of various proposals for new internet protocols
   with larger addresses.

   It is possible that CIDR will not be adequate to maintain the IP
   Internet until the long-term solutions are in place. This memo
   proposes another short-term solution, address reuse, that complements
   CIDR or even makes it unnecessary. The address reuse solution is to
   place Network Address Translators (NAT) at the borders of stub
   domains. Each NAT box has a table consisting of pairs of local IP
   addresses and globally unique addresses. The IP addresses inside the
   stub domain are not globally unique. They are reused in other
   domains, thus solving the address depletion problem. The globally
   unique IP addresses are assigned according to current CIDR address
   allocation schemes. CIDR solves the scaling problem. The main
   advantage of NAT is that it can be installed without changes to
   routers or hosts. This memo presents a preliminary design for NAT,
   and discusses its pros and cons.

Acknowledgments

   This memo is based on a paper by Paul Francis (formerly Tsuchiya) and
   Tony Eng, published in Computer Communication Review, January 1993.
   Paul had the concept of address reuse from Van Jacobson.

   Kjeld Borch Egevang edited the paper to produce this memo and
   introduced adjustment of sequence-numbers for FTP. Thanks to Jacob
   Michael Christensen for his comments on the idea and text (we thought

Egevang & Francis                                               [Page 1]
RFC 1631               Network Address Translator               May 1994

   for a long time, we were the only ones who had had the idea).

1. Introduction

   The two most compelling problems facing the IP Internet are IP
   address depletion and scaling in routing. Long-term and short-term
   solutions to these problems are being developed. The short-term
   solution is CIDR (Classless InterDomain Routing) [2]. The long-term
   solutions consist of various proposals for new internet protocols
   with larger addresses.

   Until the long-term solutions are ready an easy way to hold down the
   demand for IP addresses is through address reuse. This solution takes
   advantage of the fact that a very small percentage of hosts in a stub
   domain are communicating outside of the domain at any given time. (A
   stub domain is a domain, such as a corporate network, that only
   handles traffic originated or destined to hosts in the domain).
   Indeed, many (if not most) hosts never communicate outside of their
   stub domain. Because of this, only a subset of the IP addresses
   inside a stub domain, need be translated into IP addresses that are
   globally unique when outside communications is required.

   This solution has the disadvantage of taking away the end-to-end
   significance of an IP address, and making up for it with increased
   state in the network. There are various work-arounds that minimize
   the potential pitfalls of this. Indeed, connection-oriented protocols
   are essentially doing address reuse at every hop.

   The huge advantage of this approach is that it can be installed
   incrementally, without changes to either hosts or routers. (A few
   unusual applications may require changes). As such, this solution can
   be implemented and experimented with quickly. If nothing else, this
   solution can serve to provide temporarily relief while other, more
   complex and far-reaching solutions are worked out.

2. Overview of NAT

   The design presented in this memo is called NAT, for Network Address
   Translator. NAT is a router function that can be configured as shown
   in figure 1. Only the stub border router requires modifications.

   NAT's basic operation is as follows. The addresses inside a stub
   domain can be reused by any other stub domain. For instance, a single
   Class A address could be used by many stub domains. At each exit
   point between a stub domain and backbone, NAT is installed. If there
   is more than one exit point it is of great importance that each NAT
   has the same translation table.