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


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.


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 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.

        \ | /                 .                                /
   +---------------+  WAN     .           +-----------------+/
   |Regional Router|----------------------|Stub Router w/NAT|---
   +---------------+          .           +-----------------+\
                              .                      |         \
                              .                      |  LAN
                              .               ---------------
                        Stub border

Figure 1: NAT Configuration

For instance, in the example of figure 2, both stubs A and B internally use class A address Stub A's NAT is assigned the class C address, and Stub B's NAT is assigned the class C address The class C addresses are globally unique no other NAT boxes can use them.

                                       \ | /
                                     |Regional Router|
                                   WAN |           | WAN
                                       |           |
                   Stub A .............|....   ....|............ Stub B
                                       |           |
                     {s=,^  |           |  v{s=,
                      d=}^  |           |  v d=}
                       +-----------------+       +-----------------+
                       |Stub Router w/NAT|       |Stub Router w/NAT|
                       +-----------------+       +-----------------+
                             |                         |
                             |  LAN               LAN  |
                       -------------             -------------
                                 |                 |
               {s=, ^  |                 |  v{s=,
                d=}^ +--+             +--+ v d=}
                                |--|             |--|
                               /____\           /____\

Figure 2: Basic NAT Operation

When stub A host wishes to send a packet to stub B host, it uses the globally unique address as destination, and sends the packet to it's primary router. The stub router has a static route for net so the packet is forwarded to the WAN-link. However, NAT translates the source address of the IP header with the globally unique before the package is forwarded. Likewise, IP packets on the return path go through similar address translations.

Notice that this requires no changes to hosts or routers. For instance, as far as the stub A host is concerned, is the address used by the host in stub B. The address translations are completely transparent.

Of course, this is just a simple example. There are numerous issues to be explored. In the next section, we discuss various aspects of NAT.

3. Various Aspects of NAT

3.1 Address Spaces

Partitioning of Reusable and Non-reusable Addresses

For NAT to operate properly, it is necessary to partition the IP address space into two parts - the reusable addresses used internal to stub domains, and the globally unique addresses. We call the reusable address local addresses, and the globally unique addresses global addresses. Any given address must either be a local address or a global address. There is no overlap.

The problem with overlap is the following. Say a host in stub A wished to send packets to a host in stub B, but the local addresses of stub B overlapped the local addressees of stub A. In this case, the routers in stub A would not be able to distinguish the global address of stub B from its own local addresses.

Initial Assignment of Local and Global Addresses

A single class A address should be allocated for local networks. (See RFC 1597 [3].) This address could then be used for internets with no connection to the Internet. NAT then provides an easy way to change an experimental network to a "real" network by translating the experimental addresses to globally unique Internet addresses.

Existing stubs which have unique addresses assigned internally, but are running out of them, can change addresses subnet by subnet to local addresses. The freed adresses can then be used by NAT for external communications.

3.2 Routing Across NAT

The router running NAT should never advertise the local networks to the backbone. Only the networks with global addresses may be known outside the stub. However, global information that NAT receives from the stub border router can be advertised in the stub the usual way.

Private Networks that Span Backbones

In many cases, a private network (such as a corporate network) will be spread over different locations and will use a public backbone for communications between those locations. In this case, it is not desirable to do address translation, both because large numbers of hosts may want to communicate across the backbone, thus requiring large address tables, and because there will be more applications that depend on configured addresses, as opposed to going to a name server. We call such a private network a backbone-partitioned stub.

Backbone-partitioned stubs should behave as though they were a non- partitioned stub. That is, the routers in all partitions should maintain routes to the local address spaces of all partitions. Of course, the (public) backbones do not maintain routes to any local addresses. Therefore, the border routers must tunnel through the backbones using encapsulation. To do this, each NAT box will set aside one global address for tunneling. When a NAT box x in stub partition X wishes to deliver a packet to stub partition Y, it will encapsulate the packet in an IP header with destination address set to the global address of NAT box y that has been reserved for encapsulation. When NAT box y receives a packet with that destination address, it decapsulates the IP header and routes the packet internally.

3.3 Header Manipulations

In addition to modifying the IP address, NAT must modify the IP checksum and the TCP checksum. Remember, TCP's checksum also covers a pseudo header which contains the source and destination address. NAT must also look out for ICMP and FTP and modify the places where the IP address appears. There are undoubtedly other places, where modifications must be done. Hopefully, most such applications will be discovered during experimentation with NAT.

The checksum modifications to IP and TCP are simple and efficient. Since both use a one's complement sum, it is sufficient to calculate the arithmetic difference between the before-translation and after- translation addresses and add this to the checksum. The only tricky part is determining whether the addition resulted in a wrap-around (in either the positive or negative direction) of the checksum. If so, 1 must be added or subtracted to satisfy the one's complement arithmetic. Sample code (in C) for this is as follows:

   void checksumadjust(unsigned char *chksum, unsigned char *optr,
   int olen, unsigned char *nptr, int nlen)
   /* assuming: unsigned char is 8 bits, long is 32 bits.
     - chksum points to the chksum in the packet
     - optr points to the old data in the packet
     - nptr points to the new data in the packet
     long x, old, new;
     while (olen) {
       if (olen==1) {
         x-=old & 0xff00;
         if (x<=0) { x--; x&=0xffff; }
       else {
         old=optr[0]*256+optr[1]; optr+=2;
         x-=old & 0xffff;
         if (x<=0) { x--; x&=0xffff; }
     while (nlen) {
       if (nlen==1) {
         x+=new & 0xff00;
         if (x & 0x10000) { x++; x&=0xffff; }
       else {
         new=nptr[0]*256+nptr[1]; nptr+=2;
         x+=new & 0xffff;
         if (x & 0x10000) { x++; x&=0xffff; }
     chksum[0]=x/256; chksum[1]=x & 0xff;

The arguments to the File Transfer Protocol (FTP) PORT command include an IP address (in ASCII!). If the IP address in the PORT command is local to the stub domain, then NAT must substitute this. Because the address is encoded in ASCII, this may result in a change in the size of the packet (for instance is 12 ASCII characters, while is 14 ASCII characters). If the new size is the same as the previous, only the TCP checksum needs adjustment (again). If the new size is less than the previous, ASCII zeroes may be inserted, but this is not guaranteed to work. If the new size is larger than the previous, TCP sequence numbers must be changed too.

A special table is used to correct the TCP sequence and acknowledge numbers with source port FTP or destination port FTP. The table entries should have source, destination, source port, destination port, initial sequence number, delta for sequence numbers and a timestamp. New entries are created only when FTP PORT commands are seen. The initial sequence numbers are used to find out if the sequence number of a packet is before or after the last FTP PORT command (delta may be increased for every FTP PORT command). Sequence numbers are incremented and acknowledge numbers are decremented. If the FIN bit is set in one of the packets, the associated entry may be deleted soon after (1 minute should be safe). Entries that have not been used for e.g. 24 hours should be safe to delete too.

The sequence number adjustment must be coded carefully, not to harm performance for TCP in general. Of course, if the FTP session is encrypted, the PORT command will fail.

If an ICMP message is passed through NAT, it may require two address modifications and three checksum modifications. This is because most ICMP messages contain part of the original IP packet in the body. Therefore, for NAT to be completely transparent to the host, the IP address of the IP header embedded in the data part of the ICMP packet must be modified, the checksum field of the same IP header must correspondingly be modified, and the ICMP header checksum must be modified to reflect the changes to the IP header and checksum in the ICMP body. Furthermore, the normal IP header must also be modified as already described.

It is not entirely clear if the IP header information in the ICMP part of the body really need to be modified. This depends on whether or not any host code actually looks at this IP header information. Indeed, it may be useful to provide the exact header seen by the router or host that issued the ICMP message to aid in debugging. In any event, no modifications are needed for the Echo and Timestamp messages, and NAT should never need to handle a Redirect message.

SNMP messages could be modified, but it is even more dubious than for ICMP messages that it will be necessary.

Applications with IP-address Content

Any application that carries (and uses) the IP address inside the application will not work through NAT unless NAT knows of such instances and does the appropriate translation. It is not possible or even necessarily desirable for NAT to know of all such applications. And, if encryption is used then it is impossible for NAT to make the translation.

It may be possible for such systems to avoid using NAT, if the hosts in which they run are assigned global addresses. Whether or not this can work depends on the capability of the intra-domain routing algorithm and the internal topology. This is because the global address must be advertised in the intra-domain routing algorithm. With a low-feature routing algorithm like RIP, the host may require its own class C address space, that must not only be advertised internally but externally as well (thus hurting global scaling). With a high-feature routing algorithm like OSPF, the host address can be passed around individually, and can come from the NAT table.

Privacy, Security, and Debugging Considerations

Unfortunately, NAT reduces the number of options for providing security. With NAT, nothing that carries an IP address or information derived from an IP address (such as the TCP-header checksum) can be encrypted. While most application-level encryption should be ok, this prevents encryption of the TCP header.

On the other hand, NAT itself can be seen as providing a kind of privacy mechanism. This comes from the fact that machines on the backbone cannot monitor which hosts are sending and receiving traffic (assuming of course that the application data is encrypted).

The same characteristic that enhances privacy potentially makes debugging problems (including security violations) more difficult. If a host is abusing the Internet is some way (such as trying to attack another machine or even sending large amounts of junk mail or something) it is more difficult to pinpoint the source of the trouble because the IP address of the host is hidden.

4. Conclusions

NAT may be a good short term solution to the address depletion and scaling problems. This is because it requires very few changes and can be installed incrementally. NAT has several negative characteristics that make it inappropriate as a long term solution, and may make it inappropriate even as a short term solution. Only implementation and experimentation will determine its appropriateness.

The negative characteristics are:

1. It requires a sparse end-to-end traffic matrix. Otherwise, the NAT

   tables will be large, thus giving lower performance. While the
   expectation is that end-to-end traffic matrices are indeed sparse,
   experience with NAT will determine whether or not they are. In any
   event, future applications may require a rich traffic matrix (for
   instance, distributed resource discovery), thus making long-term use
   of NAT unattractive.

2. It increases the probability of mis-addressing.

3. It breaks certain applications (or at least makes them more difficult

to run).

4. It hides the identity of hosts. While this has the benefit of

privacy, it is generally a negative effect.

5. Problems with SNMP, DNS, ... you name it.

Current Implementations

Paul and Tony implemented an experimental prototype of NAT on public domain KA9Q TCP/IP software [1]. This implementation manipulates addresses and IP checksums.

Kjeld implemented NAT in a Cray Communications IP-router. The implementation was tested with Telnet and FTP. This implementation manipulates addresses, IP checksums, TCP sequence/acknowledge numbers and FTP PORT commands.

The prototypes has demonstrated that IP addresses can be translated transparently to hosts within the limitations described in this paper.


   [1] Karn, P., "KA9Q", anonymous FTP from
   [2] Fuller, V., Li, T., and J. Yu, "Classless Inter-Domain Routing
       (CIDR) an Address Assignment and Aggregation Strategy", RFC 1519,
       BARRNet, cisco, Merit, OARnet, September 1993.
   [3] Rekhter, Y., Moskowitz, B., Karrenberg, D., and G. de Groot,
       "Address Allocation for Private Internets", RFC 1597, T.J. Watson
       Research Center, IBM Corp., Chrysler Corp., RIPE NCC, March 1994.

Security Considerations

Security issues are not discussed in this memo.

Authors' Addresses

Kjeld Borch Egevang
Cray Communications
Smedeholm 12-14
DK-2730 Herlev

   Phone: +45 44 53 01 00

Paul Francis
NTT Software Lab
3-9-11 Midori-cho Musashino-shi
Tokyo 180 Japan

   Phone: +81-422-59-3843
   Fax +81-422-59-3765