RFC 1631 (rfc1631) - Page 1 of 10

The IP Network Address Translator (NAT)

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