A NAT is a device that allows private networks to mask their internal IP addresses, replacing them with as few as one globally unique official IP address. The box rewrites packets as they pass through the gateway.
Outbound packets have their source IP address substituted with a public identifier.
Inbound packets have their destination addresses replaced with the real target host's private IP address. 10.0.0.0 through 10.255.255.255, 172.16.0.0 through 172.31.255.255, and 192.168.0.0 through 192.168.255.255. Organizations needing IPv4 address space are finding it increasingly costly. NAT provides nearly unlimited expansion of networks without implementing a more robust and expensive IPv6 solution. The cost considerations alone ensure that NAT will remain popular.