Network Address Translators (nat)

Last Modified: 6-Feb-02


Pyda Srisuresh <>
Matt Holdrege <>

Transport Area Director(s):

Scott Bradner <>
Allison Mankin <>

Transport Area Advisor:

Scott Bradner <>

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Description of Working Group:

IP V4 Network Address Translation (NAT) has become an increasingly common function in the Internet for a variety of reasons. NATs are used to interconnect a private network consisting of unregistered IP addresses with a global IP network using limited number of registered IP addresses. NATs are also used to avoid address renumbering in a private network when topology outside the private network changes for variety of reasons. And, there are many other applications of NAT operation.

A number of NAT deployments are currently in use and naturally, a large number of internet applications work transparently with NATs. However, there are applications for which NATs would fail and custom-specific Application Level Gateways (ALGs) are required to perform translations for those applications.

NAT has the potential to interrupt end-to-end nature of Internet applications, thereby threatening end-to-end security and other end-to-end functions. In addition, NAT has topology restrictions and other constraints on the protocols and applications that run across NATs. Thus NATs have a particular area of application and should not be considered a general solution.

This working group will provide a forum to discuss applications of NAT operation, limitations to NAT, and impact of NAT operation on internet protocols and applications. The Working Group recognizes that NAT may interfere with protocols that use cryptographic protection for authentication, integrity or confidentiality. The Working Group will NOT suggest changes in such protocols to make them NAT friendly when such modification will significantly reduce the security provided by those protocols. However, the Work Group will examine and discuss alternative solutions, and other new ideas relating to this issue. Broadly speaking, the objective of the work group is to come up with a series of documents in the following categories.

The first category of documents will address what NAT is, how NAT works and applications of NAT operation in address conservation, prevention of address renumbering, load sharing and other areas.

The second category of documents will address requirements of NAT and limitations to NAT operation. Specifically, this will include a detailed list of applications which are known to have problems working over NATs.

The third category of documents are Informational RFCs which will specify NAT friendly application and protocol design guidelines, interactions between NATs and applications such as DNS and protocols such as IP sec. Particular emphasis will be placed on security issues. The Work group will also examine and discuss various alternative solutions, and other ideas to identify areas where NATs or other protocols and applications can be improved to overcome the shortcomings in interoperability or functionality.

The fourth category of documents will deal with network management of NATs.

Exploration of the use of NATs for load sharing is not within the scope of this working group.

The goals and milestones section below lists just the subject matter of issues to be covered. This is done so deliberately because there may be some adjustments made to the packaging of the RFCs, while covering all of the contents below. The work group will decide how to group the contents into different RFCs.

Goals and Milestones:

Done &nbsp&nbsp Submit Internet-Draft on what is NAT and how NAT works.
Done &nbsp&nbsp Submit Internet-Draft on NAT limitations and a list of applications and protocols known to have problems working with NAT.
Done &nbsp&nbsp Submit Internet-Draft on NAT friendly application and protocol design guidelines.
Done &nbsp&nbsp Submit Experimental RFC on Realm-Specific IP (RSIP) framework
Done &nbsp&nbsp Submit Experimental RFC on Realm-Specific IP (RSIP) protocol specificatio
Done &nbsp&nbsp Submit RFC on RSIP Support for End-to-end IPsec
Done &nbsp&nbsp Submit Informational RFC on NAT friendly application design guidelines
Done &nbsp&nbsp Submit Informational RFC on Framework for interfacing with NAT
Nov 01 &nbsp&nbsp Submit Internet-Draft on Network Management Information Base for NATs.

Request For Comments:

IP Network Address Translator (NAT) Terminology and Considerations (RFC 2663) (72265 bytes)
DNS extensions to Network Address Translators (DNS_ALG) (RFC 2694) (67720 bytes)
Security Model with Tunnel-mode IPsec for NAT Domains (RFC 2709) (24552 bytes)
An SNMP Application Level Gateway for Payload Address Translation (RFC 2962) (46803 bytes)
Traditional IP Network Address Translator (Traditional NAT) (RFC 3022) (37675 bytes)
Protocol Complications with the IP Network Address Translator (NAT) (RFC 3027) (48662 bytes)
Realm Specific IP: A Framework (RFC 3102) (73856 bytes)
Realm Specific IP: Protocol Specification (RFC 3103) (115855 bytes)
RSIP Support for End-to-end IPSEC (RFC 3104) (38719 bytes)
Finding an RSIP Server with SLP (RFC 3105) (21427 bytes)
NAT Friendly Application Design Guidelines (RFC 3235) (29588 bytes)