Network address translation

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 initially 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.

Source: Wikipedia — Network address translation (CC BY-SA 4.0)