(NAT, or Network Address Translator, Virtual LAN) A technique in which a router or firewall rewrites the source and/or destination Internet addresses in a packet as it passes through, typically to allow multiple hosts to connect to the Internet via a single external IP address. NAT keeps track of outbound connections and distributes incoming packets to the correct machine.
NAT is an alternative to adopting IPv6 (IPng). It allows the same IP addresses (10.x.x.x is the conventional range) to be used on many private local networks while requiring only one of the increasingly scarce public addresses to be allocated to each private network.
NAT does not however allow an external service to initiate a TCP connection to an internal host, nor does it support stateless protocols based on UDP well unless the router software has extensions to support each specific protocol.