RFC 1631 (rfc1631) - Page 2 of 10

The IP Network Address Translator (NAT)

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.




Egevang & Francis