(gnat) abbr. An IP routing hack used by DSL/cable routers and other gateways to make all traffic to and from a LAN of computers behind them appear as if it refers to a single IP address on the WAN. Short for Network Address Translation. Formerly called IP spoofing. RFC 1631. The technique basically revolves around having the router modify all of the packets from a client on the LAN to a server on the WAN so they appear to come directly from the router’s WAN IP address. Each such transaction results in the creation of a fake port number. When the response arrives at the router from the server, the port number is used to decode the true destination and to route the packet onto the LAN properly.