Static and Dynamic Address Mapping
The NAT can use either static or dynamic mapping. A static mapping is configured so that traffic is always mapped a specific way. You could map all traffic to and from a specific private network location to a specific Internet location. For instance, to set up a Web server on a computer on your private network, you create a static mapping that maps [Public IP Address, TCP Port 80] to [Private IP Address, TCP Port 80].
Dynamic mappings are created when users on the private network initiate traffic with Internet locations. The NAT automatically adds these mappings to its mapping table and refreshes them with each use. Dynamic mappings that are not refreshed are removed from the NAT mapping table after a configurable amount of time. For TCP connections, the default time-out is 24 hours. For UDP traffic, the default time-out is one minute.