About Network Address Translation

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The number of available IP addresses in the current 32–bit format is not enough to meet the growth in demand for access to the Internet. Network Address Translation (NAT) was designed as a short-term solution to this problem by providing a method of connecting multiple computers to an IP network using either a single unique public IP address, or a small number of unique public IP addresses.

NAT is commonly used in corporations, where a NAT router sits at the edge of the private network (referred to in this context as a stub domain) and translates the IP addresses attached to packets entering and leaving the stub domain. The NAT router, which effectively acts as an agent between the Internet and the local network, maintains a list of the mappings between public and private addresses.

Note: A stub domain is a local network using internal IP addresses. The network can use unregistered, private, IP addresses for internal communication—these addresses must be translated into unique, public, IP addresses when communicating outside the network. The addresses used internally by a given stub domain can also be used internally by any other stub domain.

For example, when a computer within the private network requests information from the public network, the NAT router automatically translates the private address of that computer into the public address of the domain, which is the only address that is transmitted to the public network. When the requested information is returned, the NAT router consults its internal list of public to private address mappings in order to forward the information to the appropriate computer.

There are a number of different ways to configure a NAT environment. The following descriptions detail the most common types of NAT environment.