The key part from Imanage's excellent link...
Quote:
Here's how dynamic NAT works:
*An internal network (stub domain) has been set up with IP addresses that were not specifically allocated to that company by IANA (Internet Assigned Numbers Authority), the global authority that hands out IP addresses. These addresses should be considered non-routable since they are not unique.
* The company sets up a NAT-enabled router. The router has a range of unique IP addresses given to the company by IANA.
* A computer on the stub domain attempts to connect to a computer outside the network, such as a Web server.
* The router receives the packet from the computer on the stub domain.
* The router saves the computer's non-routable IP address to an address translation table. The router replaces the sending computer's non-routable IP address with the first available IP address out of the range of unique IP addresses. The translation table now has a mapping of the computer's non-routable IP address matched with the one of the unique IP addresses.
* When a packet comes back from the destination computer, the router checks the destination address on the packet. It then looks in the address translation table to see which computer on the stub domain the packet belongs to. It changes the destination address to the one saved in the address translation table and sends it to that computer. If it doesn't find a match in the table, it drops the packet.
* The computer receives the packet from the router. The process repeats as long as the computer is communicating with the external system.
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