This is the most common type of NAT and has become synonymous with the term "NAT" in common usage. Other names include port address translation (PAT), IP masquerading, NAT overload and many-to-one NAT. RFC 2663 uses the term network address and port translation (NAPT) for this type of NAT. For these protocols, the port numbers are changed so that the combination of IP address (within the IP header) and port number (within the Transport Layer header) on the returned packet can be unambiguously mapped to the corresponding private network destination. The vast bulk of Internet traffic uses Transmission Control Protocol (TCP) or User Datagram Protocol (UDP). To avoid ambiguity in how replies are translated, further modifications to the packets are required. 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. Īll IP packets have a source IP address and a destination IP address. When the router receives inbound traffic from the Internet, it uses the connection tracking data it stored during the outbound phase to determine to which private address (if any) it should forward the reply. The router tracks basic data about each active connection (particularly the destination address and port).
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