Some modems or gateways have the ability to be configured with multiple IPv4 addresses for each of their ethernet ports. However, because of the declining number of public IPv4 addresses, the ability to get them for residential accounts has been restricted.
To find out if your area still offers multiple IP addresses, please do the following:
Check near the bottom for the line stating "Additional IP address" as shown below (note the prices may be different 😞
If the option is there, post a request on the forum, specifically stating that you want an additional IP address, and that you've checked your rate card, and that it's available. A Comcast Official Employee will assist you from there.
If the option is not there, you will not be able to add an additional IP address.
Please also note that even if your area has stopped offering multiple IP addresses, any previous add-on will be grandfathered in for as long as there are no changes made to the plan, account, or the equipment. Doing so may cause you to lose the additional IP address.