I don't have cable; I have DSL, but they both work on the same principle. All you get from the cable modem (ISP, actually) is your IP and connectivity) If you had a hub/router/switch whatever and had the cable modem feeding into the input of the device, and then 2 or more pc's connect to the device, you should be able to see either computer from either computer, unless you had it configured wrong, or had it configured that one PC was the server and others were just "dumb nodes". All the cable modem does is feed a signal. PC's seeing each other have nothing to do with the modem. You may have a problem that needs fixing, but it's got nothing to do w/the modem.
Networking gurus please correct me if I'm wrong. I'm not the networking guru (i wish) but I understand the concepts...for the most part.