Yeah, it's the same for every cable modem (every NTL/Ambit cable modem, that is).
192.168.x.x addresses aren't actually internet IPs, they're local addresses - 192.168.100.1 is the 'site' which is hosted by the cable modem itself. The cable modem, in that capacity, is acting as its own little server.
Routers can do the same sort of thing, allowing you to change their settings.
