RFC 2663 (rfc2663) - Page 2 of 30

IP Network Address Translator (NAT) Terminology and Considerations

RFC 2663           NAT Terminology and Considerations        August 1999


1. Introduction and Overview

   The need for IP Address translation arises when a network's internal
   IP addresses cannot be used outside the network either because they
   are invalid for use outside, or because the internal addressing must
   be kept private from the external network.

   Address translation allows (in many cases, except as noted in
   sections 8 and 9) hosts in a private network to transparently
   communicate with destinations on an external network and vice versa.
   There are a variety of flavors of NAT and terms to match them. This
   document attempts to define the terminology used and to identify
   various flavors of NAT. The document also attempts to describe other
   considerations applicable to NAT devices in general.

   Note, however, this document is not intended to describe the
   operations of individual NAT variations or the applicability of NAT
   devices.

   NAT devices attempt to provide a transparent routing solution to end
   hosts trying to communicate from disparate address realms. This is
   achieved by modifying end node addresses en-route and maintaining
   state for these updates so that datagrams pertaining to a session are
   routed to the right end-node in either realm. This solution only
   works when the applications do not use the IP addresses as part of
   the protocol itself. For example, identifying endpoints using DNS
   names rather than addresses makes applications less dependent of the
   actual addresses that NAT chooses and avoids the need to also
   translate payload contents when NAT changes an IP address.

   The NAT function cannot by itself support all applications
   transparently and often must co-exist with application level gateways
   (ALGs) for this reason. People looking to deploy NAT based solutions
   need to determine their application requirements first and assess the
   NAT extensions (i.e., ALGs) necessary to provide application
   transparency for their environment.

   IPsec techniques which are intended to preserve the Endpoint
   addresses of an IP packet will not work with NAT enroute for most
   applications in practice. Techniques such as AH and ESP protect the
   contents of the IP headers (including the source and destination
   addresses) from modification. Yet, NAT's fundamental role is to alter
   the addresses in the IP header of a packet.

2. Terminology and concepts used

   Terms most frequently used in the context of NAT are defined here for
   reference.



Srisuresh & Holdrege         Informational