Inside the configuration file that each modem receives on startup, is a field called CPE=x. This value dictates exactly how many mac addresses behind the modem are allowed to pull a real, routable, Comcast IP address. This is also what they change when you purchase another IP, since it lets you pull more than one.
The solution to this problem is quite easy, simply power-cycle the modem.
Keep in mind that if you have a router, and previously had a PC connected, you will need to restart the modem, or clone the mac of the PC into the router. Also, the router only counts as that one device and not the devices behind the router.
*This FAQ is based on user knowledge from a volunteer core of BroadbandReports' members. This FAQ in no way constitutes official information from Comcast or any of its affiliates.