IPv6 Network Administration

NAT

Network Address Translation (NAT) is a technique that has arisen in response to the shortage of globally routable IPv4 addresses. It allows a single IP address to provide connectivity for a large number of hosts. The usual deployment of NAT involves a network of hosts using one of the private address ranges mentioned in the Section 1.1 earlier in this chapter, routing traffic to a gateway or proxy with a private IP address on the inside of the network, and a real IP address on the outside facing the Internet. For outgoing traffic, this gateway replaces the private IP address with its public IP address and uses port numbers to remember the private IP address to which replies should be directed.

For incoming traffic, the gateway looks up the port numbers in a table, the original private IP address is determined and the packet is forwarded to the host in the private network. The details of traditional NAT are discussed in RFC 3022.

Because of the economics of IPv4 addresses, NAT has proven extremely popular with small businesses and home users. While one cannot strictly speaking sell IP addresses, ISPs commonly attach a charge for routing and other services to make these addresses useful. This cost is often prohibitive for many users of the SOHO persuasion. NAT has provided a way to connect an entire office to the Internet using a single, cheap dialup connection - it might even be cheerful if you're lucky.

NAT has also been used^[3] by some organizations as a form of protection against ...