NAT allows the addresses used within your LAN to be replaced by a different address when connecting to an external service.

Typically a service provider will allocate you a single IP address to be used when connecting to their service. NAT allows all your user's traffic to appears to be coming from that single address without having to change any of your user's real addresses. This is useful as internally most networks use addresses that have been reserved for public use within networks but are not valid for routing across the internet (since the same addresses may be being used on other networks). Also as stated it allows multiple users to use the same service simultaneously.

The use of NAT is automatically enabled if the system Service being used includes an IP address that is not in the same domain as the its LAN1 IP address.

An exception to the above applies for systems with two LAN's, LAN1 and LAN2. For these units, on each LAN, Enable NAT can be selected and then applied to traffic between the two LAN's.