IP address
An Internet Protocol address (IP address) is a numerical label assigned to each device connected to a computer network that uses the Internet Protocol for communication.
An IP address serves two principal functions: host or network interface identification and location addressing.
Network address translation
All IP packets have a source IP address and a destination IP address. Typically packets passing from the private network to the public network will have their source address modified, while packets passing from the public network back to the private network will have their destination address modified. To avoid ambiguity in how replies are translated, further modifications to the packets are required. The vast bulk of Internet traffic uses Transmission Control Protocol (TCP) or User Datagram Protocol (UDP). For these protocols the port numbers are changed so that the combination of IP address and port information on the returned packet can be unambiguously mapped to the corresponding private network destination. RFC 2663 uses the term "network address and port translation" (NAPT) for this type of NAT. Other names include "port address translation" (PAT), "IP masquerading", "NAT overload" and "many-to-one NAT". This is the most common type of NAT and has become synonymous with the term "NAT" in common usage.