In some networks, most, if not all, IP hosts need to reach the Internet. If that network uses private IP
addresses, the NAT router needs a very large set of registered IP addresses. If you use static NAT, each
private IP host that needs Internet access needs a publicly registered IP address. Dynamic NAT lessens the
problem, but if a large percentage of the IP hosts in the network need Internet access throughout normal
business hours, a large number of registered IP addresses would also be required. These problems can be
overcome through overloading with port address translation. Overloading allows NAT to scale to support
many clients with only a few public IP addresses.
To support lots of inside local IP addresses with only a few inside global, publicly registered IP addresses,
NAT overload uses Port Address Translation (PAT), translating the IP address as well as translating the port
number. When NAT creates the dynamic mapping, it selects not only an inside global IP address but also a
unique port number to use with that address. The NAT router keeps a NAT table entry for every unique
combination of inside local IP address and port, with translation to the inside global address and a unique
port number associated with the inside global address. And because the port number field has 16 bits, NAT
overload can use more than 65,000 port numbers, allowing it to scale well without needing many registered
IP addresses.