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# Understanding Address Translation

Address translation can occur one of two ways: inside or outside source translation.

## Inside Source Translation

Inside source translation is the most commonly used NAT configuration. When an inside host sends a packet to the outside network, the NAT router translates the source information (either the source address or the source address/port pair) and, in the inbound direction, restores the original information (this time operating on the destination address or address/port pair).

For outbound traffic, the NAT router translates the inside local address (or address/port) into the inside global address (or address/port), either through a statically defined translation or dynamically created translation. For inbound traffic, a translation must be found to revert the inside global address (or address/port) into the inside local address (or address/port), or the packet is not routed into the inside network.

| 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 used in NAT configurations only when addresses of external hosts might create a conflict on the private network. This complementary translation process is performed on the opposite addressing fields in the IP packet. When an outside host sends a packet to the inside network, the NAT router translates the source information (either the source address or the source address/port pair) and, in the outbound direction, restores the original information (this time operating on the destination address or address/port pair).

For inbound traffic, the NAT router translates the outside global address (or address/port) into the outside local address (or address/port), either through a statically defined translation or dynamically created translation. For outbound traffic, a translation must be found to revert the outside local address (or address/port) into the outside global address (or address/port), or the packet is not routed into the outside network.

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

You use outside source translation along with inside source translation to configure twice NAT.

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