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Network address port translation (NAPT) is a technique in which port numbers and private Internet Protocol (IP) addresses are mapped from multiple internal hosts to one public IP address.
It is a type of network address translation (NAT) technology that extends capabilities by translating and mapping port numbers, in addition to the IP address, when communicating with an external network.
NAPT maps Transmission Control Protocol/User Datagram Protocol (TCP/UDP) port numbers and IP addresses of local devices with registered, public IP addresses and corresponding TCP/UDP ports. NAPT uses the same public IP address to enable the connection of multiple internal hosts with external networks or the Internet.
NAPT is usually configured on a routing device, like a router. Each internal host requesting connection to an external network passes through the NAPT device, which changes the host device's IP address and port number to a pool of public IP addresses and port numbers. For example, a device that requests access may have the IP address 10.0.0.1 and port 5678. When its packets leave the NAPT device, its IP address is 126.96.36.199 - or any available public IP address - and port 7650.
The NAT device or router usually is configured with or assigned the public IP address, allowing multiple local hosts to have the same IP address with different port numbers in order to access Internet/external networks.