You need to post a lot more details about your setup before you can get a comprehensive solution. We have no idea what your modem's make or model is, whether it's a plain modem or a gateway, if you have a router installed, etc.
I am not a Comcast Employee. I am a Customer Expert volunteering my time to help other customers here in the Forums. We ask that you post publicly so people with similar questions may benefit from the conversation. Was your question answered? Mark the post as Best Answer!