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First off, the NAT device has to have a unique IP address to identify the host to the internet. Once you are behind the firewall, you can use whatever IP address you want. One the way outbound across the firewall, the internal IP addresses are converted to the public IP address, and inbound packets result in the NAT device converting in the opposite way. The main benefit is that NAT slows down the speed at which IP address space is assigned because a single IP address can be used for more than a single host. Outgoing and inbound packets are kept track of by the NAT device and matched up so the incoming packets match up with the correct host by switching destination addresses as necessary.