basic network experience dictates to me that Comcast will give you multi Ip through same cable modem.
Would be based on MAC addresses... The example is anytime you change or switch a machine that in on a cable modem it gives the ISP your one allocated MAC address. When you buy additional IP's they make your MAC pool Higher say depending on how many you buy. Honestly, it comes down to what you need. Is the extra money worth what you can do for free on a NAT'd Network?
this isnt exactly what you asked but is pretty much going to cover a general base. Since thats how most ISP will run their networks. It would cost to much for ISP to have you use two cable modems rather its easier for them to use TCP/IP protocol to handle it through 1 cable modem.
Can't say to much but with a little snooping.. you can actually login to your cable modem and find out whats its doing.
Most of them are setup to use 192.168.100.1 try that in your web browser and see what shows up. Not all of them use this ip but I think over 70 % do