Understanding Address Translation

Address translation can occur under one of two general methods — inside or outside source translation.

Inside Source Translation

Inside source translation is the most commonly used in NAT configuration. In this method, the NAT router performs translation between an inside (private) network address and an outside network address (for example, the Internet). The NAT router replaces the private source address with a globally unique address (either statically or dynamically). When the external destination responds, the NAT router, after successfully locating the address translation in its translation table, "untranslates" the global address (now the destination address) back to the local, private address.

Note: Dynamic inside source translations are established by outbound traffic.

You use inside source translation in traditional and bidirectional NAT configurations.

Outside Source Translation

Outside source translation is less often used in NAT configuration because you are configuring the translation of an outside source to access an inside network. Normally, outside hosts have globally unique IP addresses, making translation unnecessary.

You use outside source translation along with inside source translation to configure translation in both directions, from both sides of the NAT router (not just translation of outbound information and "untranslation" of returning information).

Note: Dynamic outside source translations are established by inbound traffic.

Cases that require this type of translation method occur in twice NAT configurations that set up translation between two networks that contain nonunique or not publicly routable IP addresses (for example, two separate networks that use overlapping IP address blocks).