Cisco ASA and PIX Firewall Handbook

11-2. Configuring Network Address Translation

You can use Network Address Translation (NAT) to translate the IP address of a host on one IOS firewall interface to a different address on another interface. This is useful for conserving IP address space, as well as for isolating or hiding internal hosts behind public IP addresses so that they don't have a presence on the public network.

Address translation is triggered by packets moving across NAT boundaries. The boundaries are marked by an inside and outside NAT interface. For example, when a packet arrives on an inside NAT interface and is forwarded on an outside NAT interface, address translation is triggered. The reverse is also true—a packet arriving on the outside NAT interface triggers address ...