Core Issue
Port Address Translation (PAT) (also called Network Address Translation (NAT), Network Address Port Translation (NAPT) or IP masquerading) overloading allows many inside local addresses to be mapped to a single inside global address at the same time. This method is useful when few valid IP addresses are available. However, there are many hosts on the inside network that need to communicate with the outside network.
Resolution
When PAT is configured, the router uses the same inside global address to translate multiple inside local addresses, and it makes use of the source port numbers to distinguish the sessions from different hosts. PAT creates an extended translation entry in the NAT table by including the protocol, as well as the port information. Since the source port number used for maintaining the uniqueness of a session can have a value from 0 to 65535, there can be a theoretical maximum of 65536 PAT entries at a time for each inside global address. Since each NAT entry requires about 160 bytes of memory, the number of translations possible is limited by the amount of memory available on the router.
For related information, refer to NAT Frequently Asked Questions.
