11.2 NAT Operation
11.2.3 Translating inside local addresses
The Figure illustrates NAT operation when it is used to translate addresses from inside your network to destinations outside of your network.

The steps in the following list correspond to the numbered NAT operation steps in the Figure:

  1. User at host 10.1.1.1 opens a connection to outside host B.
  2. The first packet that the border router receives from host 10.1.1.1 causes the router to check its NAT table.
    If a translation is found because it has been statically configured, the router continues to Step 3. If no translation is found, the router determines that address 10.1.1.1 must be translated. The router allocates a new address and sets up a translation of the inside local address 10.1.1.1 to a legal inside global address from the dynamic address pool. This type of translation entry is referred to as a simple entry.
  3. The border router replaces the 10.1.1.1 inside local IP address with the selected inside global address, 192.168.2.2, and forwards the packet.
  4. Host B receives the packet and responds to that node by using the inside global IP address 192.168.2.2.
  5. When the border router receives the packet with the inside global IP address, the router performs a NAT table lookup by using the inside global address as the reference. The router then translates the address to the 10.1.1.1 inside local address and forwards the packet to 10.1.1.1. Host 10.1.1.1 receives the packet and continues the conversation. For each packet, the router performs Steps 2 through 5.