There was a great guide posted here a few months back (at least I think it was this site) It was a link to a powerpoint slideshow.
Sadly I can't find it but someone else might have better luck. It explained the basics on how cable modems shared an area. I'll keep looking and if I find it I'll post a link.
EDIT - Found it tucked away. Might be a bit technical but gives the basic points.
http://homepage.ntlworld.com/draig.goch/