How NAT Works

Okay, now it’s time to look at how this whole NAT thing works. I’m going to start by using Figure 13-2 to describe the basic translation of NAT.

In the example shown in Figure 13-2, host 10.1.1.1 sends an outbound packet to the border router configured with NAT. The router identifies the source IP address as an inside local IP address destined for an outside network, translates the source address, and documents the translation in the NAT table.

The packet is sent to the outside interface with the new translated source address. The external host returns the packet to the destination host and the NAT router translates the inside global IP address back to the inside local IP address using the NAT table. This is as simple as it gets.