If you have a Comcast provided modem/router, and you are only connecting one computer to it via a hardwired Ethernet connection (not wifi), then you may be able to log into the web interface for the device (try http://192.168.100.1) and see if you can find a setting for bridge mode. If you can that's what you want to do, your computer will the obtain the public IP rather than a private IP.
If your Comcast provided equipment does not have a Bridge Mode setting, it is possible to get Comcast to set this for you. I wouldn't take the 1-800 number as the route to request it tho, the calltakers usually have no concept of what you're asking for. Instead use the Comcast Support forum and PM one of the mods, or try twitter @ComcastCares
Not part of your question, but advise I'll give anyway: If you have a Comcast owned internet modem or internet/phone device, stop paying the rental fee and buy your own. The payback is 9 months to 2 years depending on your choices. With phone service there are fewer choices, but the Arris combo units are available on amazon. With no phone involved just buy a DOCSIS 3.0 (or 3.1) cable modem and if needed a separate router. (I prefer separate devices because troubleshooting and upgrading over time are less complicated.)