I am having the same trouble since I moved to Comcast Modem Router from Qwest. I also have a WAP device that has the same problem when working as a wireless network card. It seems like the router in the Comcast modem router blocks these connections but doesn't show you where or why or give you access.
Could someone from Comcast please help. I tried calling and all they do is check that the modem is up at their end and then try and pass you off to the Signature service for a fee. This is something that should be covered by Comcast themselves since the issue is with their router not our devices.
The only workaround I can see is to ask them to put your router into bridge mode and buy or use an third party router that you have control over. I have the same trouble with Parallels. I got my device running through a WAP configured to be a bridge by putting its IP in the Comcast router DMZ but that is not advisable as it is insecure and unprotected by the firewall.