I remember when we got Internet connectivity. The Banyan winsock was... interesting. Not PAT in the traditional sense - when you bound a listening IP port, it bound that port on the server that actually *had* the IP address.
What that boiled down to was: If you configured your desktop to bind a listening server on TCP port 80 for instance, if someone hit your public IP on port 80 they would get *your computer*. As long as nobody else bound that port first. If they did, you'd get an error when you'd try to bind it.