Port address translation (PAT) is a network device feature that allows multiple devices on a private network to share a public network address. It modifies incoming and outgoing IP packets, creating a table that maps addresses and port numbers. PAT is a subset of network address translation (NAT) and is used by routers to preserve public IP addresses.
Port address translation is a means for multiple devices on a private network to share a public network address. It is a network device feature that modifies incoming and outgoing Internet Protocol (IP) packets. The IP address and port number are both mangled, preventing the public network from directly accessing hosts on the private network. Port address translation is used by network routers to share a public Internet address with all computers on the local network. This preserves public IP addresses instead of requiring one for each local computer that needs to access the Internet.
Both Transmission Control Protocol (TCP) and User Datagram Protocol (UDP) packets are translated by this process. Each packet contains a source IP address and a source port number. It also includes a destination IP address and destination port number. Port address translation, also known by the acronym PAT, creates a table that maps addresses and port numbers as they are used. This table is maintained in the PAT device, which can be a hardware router, software firewall, or similar device.
When a computer on the local network sends a packet to the public network, the PAT device can alter its header data. The actual source address is replaced with the IP address of the port address translation device. A new port number is assigned by the PAT device for connecting the source computer to the public network. The actual source port number in the header is then replaced with the newly assigned one. This modifies the outgoing packet to appear as if it came directly from the PAT device.
For an outgoing packet, the port address translation table keeps a copy of the source address, source port, and assigned port. The same port number can be used for any packet with that source address and port, simply by performing a table lookup. When the public network responds, it will send packets to the IP address of the PAT device on the assigned port number. The PAT device will then replace that destination address with the actual private address from the lookup table. It will also replace the port number in the packet header with the actual port number from the table.
Network address translation (NAT) is a related method of mapping IP addresses. Port address translation is actually a subset of NAT called overloaded NAT. PAT is sometimes referred to as NAT or network address port translation. There are variants of NAT that map only IP addresses or only source addresses and source port numbers. Other modules map only destination addresses and destination port numbers or both types of ports and addresses.
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