What is «NAT»?

Network address translation (NAT) is a method of mapping an IP address space into another by modifying network address information in the IP header of packets while they are in transit across a traffic routing device.

The technique was originally used to bypass the need to assign a new address to every host when a network was moved, or when the upstream Internet service provider was replaced, but could not route the network's address space.
It has become a popular and essential tool in conserving global address space in the face of IPv4 address exhaustion.
One Internet-routable IP address of a NAT gateway can be used for an entire private network.