Unicast IP Routing

Network Address Translator

A Network Address Translator (NAT) is an IP router defined in RFC 1631 that can translate IP addresses and TCP/UDP port numbers of packets as they are being forwarded. Consider a small business network with multiple computers connecting to the Internet. A small business would normally have to obtain an Internet Service Provider (ISP)–allocated public IP address for each computer on their network. With the NAT, however, the small business can use private addressing (as described in RFC 1597) and have the NAT map its private addresses to a single or to multiple public IP addresses as allocated by its ISP.

For example, if a small business is using the 10.0.0.0 private network for its intranet and has been granted the public IP address of 198.200.200.1 by its ISP, the NAT maps (using static or dynamic mappings) all private IP addresses being used on network 10.0.0.0 to the public IP address of 198.200.200.1.

When a private user on the small business intranet connects to an Internet resource, the user's IP stack creates an IP packet with the following values set in the IP and TCP or UDP headers (bold text indicates the fields changed by the NAT):

Destination IP Address: Internet resource IP address
Source IP Address: Private IP address
Destination Port: Internet resource TCP or UDP port
Source Port: Source application TCP or UDP port

The source host or another router forwards this IP packet to the NAT, which translates the addresses of the outgoing packet as follows (bold text indicates the fields changed by the NAT):

Destination IP Address: Internet resource IP address
Source IP Address: ISP-allocated public address
Destination Port: Internet resource TCP or UDP port
Source Port: Remapped source application TCP or UDP port

The NAT sends the remapped IP packet over the Internet. The responding computer sends back the response to the NAT. When received by the NAT, the packet contains the following addressing information (bold text indicates the fields changed by the NAT):

Destination IP Address: ISP-allocated public address
Source IP Address: Internet resource IP address
Destination Port: Remapped source application TCP or UDP port
Source Port: Internet resource TCP or UDP port

When the NAT maps and translates the addresses and forwards the packet to the intranet client, it contains the following addressing information (bold text indicates the fields changed by the NAT):

Destination IP Address: Private IP address
Source IP Address: Internet resource IP address
Destination Port: Source application TCP or UDP port
Source Port: Internet resource TCP or UDP port

For outgoing packets, the source IP address and TCP/UDP port numbers are mapped to a public source IP address and a possibly changed TCP/UDP port number. For incoming packets, the destination IP address and TCP/UDP port numbers are mapped to the private IP address and original TCP/UDP port number.