Network Address Translation

Network Address Translation (NAT) allows an organization to use private IP address space inside the organization (or any other IP address it might require) and present this IP address differently to the outside networks. Organizations might use NAT for the following purposes:

■ To connect private IP internetworks that use nonregistered IP addresses to the Internet, NAT translates the internal local addresses to globally unique IP addresses before sending packets to the outside network.
■ Internal addresses must be changed, and this creates a large administrative burden. NAT is used instead to translate addresses.
■ To do basic load sharing of TCP traffic. A single global IP address is mapped to many local IP addresses by using the TCP load distribution feature.

NAT uses the following definitions:

■ Inside local address—The IP address that is assigned to a host on the inside network. Often, this is a nonregistered IP address.
■ Inside global address—A legitimate IP address that represents one or more inside local IP addresses to the outside world.
■ Outside local address—The IP address of an outside host as it appears to the inside network.
■ Outside global address—The IP address assigned to a host on the outside network by the owner of the host.