You can assign additional IPs, but I believe the limitation is that they have to be on the same subnet, as they do not allow for any additional settings. Now, that being said, as long as you use an address range with the same subnet mask, you should be fine, as you wont need a default gateway. There are serious problems with this, though. Firewalls won't work well because they can easily be circumvented by spoofing the internal addresses from the outside. I suggest setting up a dedicated computer with 2 NICs, or buying a piece of hardware like the Linksys Router/Firewall box.