The Hidden Network

Because the application level gateway re-addresses all authorized packets, all computers on the protected network become invisible to computers on the Internet. The protected network is now a hidden network. The firewall is the only computer that can be seen from the Internet.

Completely hiding the protected network provides an extra layer of security beyond that of filtering. It is more difficult to hack a network that doesn't exist. The hacker can know nothing about the protected network, without first cracking the firewall computer itself.