mmmm not heard of an 86.xx ip address before
Set top box Broadband and Cable Modem are registered quite differently.
With the set top box Broadband it's the PC that is registered onto the system so you can't plug any old pc into it, you have to register it before it will work. This uses the address "start.ntl" to load the registration page and asks you to "name" the pc.
With the Cable Modem it is the Modem that's registered, not the pc using address "autoreg.autoregister.net" to register so you can swop and change pc's as much as you like so long as you reboot the modem each time.