NAT (Network Address Translation) is a process that maps one set of IP addresses to another set of IP addresses. NAT is commonly used to map a fixed set of addresses within a network to a smaller or variable set of addresses used externally. For example a large commercial Web site, might handle requests to a single IP address on the Internet with a network of machines that have their own internal set of IP addresses (fixed or dynamic) never seen by the outside world. Or, a facility might use fixed IP addresses internally, but obtain it's external IP address dynamically via DHCP.

NAT is sometimes used as a Security tool and is often implemented in Security firewalls since machines on one side of the NAT logic can only address messages to those on the other as allowed by the NAT mappings. Some NAT implementations also map FTP and other ports in much the same way as NAT maps IP addresses. NAT is defined in RFC 1631

Use of NAT can have unexpected side affects. For example it may not be possible for two players to log into the same on-line game from different PCs if both are communicating to the game server through the same NAT interface.

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