This post should help those struggling to realize the concept of having multiple external IP addresses registered to a single modem.
I’m currently in the process (after a year or so of waiting for my employer’s to get ready for this) of setting something like this up to have two web servers behind one Comcast modem.
We will be hosting an Exchange Server and a Moodle site from two separate servers using just one modem.
You may or may not have the same setup as this guy, but the poster’s give the basics of what you need and how to go about this type of setup.
I have completed my setup at my employer’s location. I replaced their sonicwall as the router and put in an Untangle box as the replacement. That made it easy to setup the additional IP’s as aliases, and create forwarders for the additional Internet IPs > Internal IP’s.
Now, they have two fully functioning websites running on a single cable modem using SNAT or 1:1 NAT. It was quite easy once I had the IP’s actually configured on the modem.