The Free On-line Dictionary of Computing (18 March 2015):
Network Address Translation
Network Address Translator
(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.