DOCSIS cable modems, once they've acquired upstream and downstream signals, send a DHCP DISCOVER broadcast packet. The CMTS they've ranged with forwards the broadcast to a DHCP server; The server will check the MAC address (or what other uniquely identifying attribute or attributes the MSO chooses) against its DHCP hosts table, which is usually populated by modem provisioning software. If present, then the server will issue a DHCP OFFER back to the CMTS. Depending on how the ISP has implemented its modem provisioning, the DHCP server may offer the cable modem a specific configuration (IP address, netmask, default gateway, time server, log server, configuration file server and filename, and so forth) that may no longer work if the hybrid fiber/coaxial segment is connected to a different CMTS, or even a different upstream/downstream port on the same CMTS. If the cable modem receives a DHCP OFFER and receives configuration information that does not work, it cannot complete the DHCP registration process, and will, after a time, begin the re-ranging process, in case there are more than one CMTS in operation on that cable plant.
If the location where you're taking your modem already has cable modem service, and is not too far away from the original service address, then unless you happen to live at a physical edge of a cable node, your cable modem will likely work at the near-by address. If you're taking the modem to a more distant site, or the address does not have cable modem service, then the cable company might have put filters on the line to limit the amount of ingress noise received by the CMTS on the upstream channels it uses. While either the cable modem will work or it won't at another address, the reason why it isn't working can be determined by watching the cable modem's lights, and noting which lights hold steady and which lights are still blinking when the modem resets (all lights go out) and starts re-ranging (downstream light blinks). However, I recommend against calling the cable company if a device that was installed at one address does not work at another address, for any reason other than a billing change.
Should your cable modem successfully come up at another address, it would be difficult for the cable company to determine that the modem was not at its service address, but since the cable modem is assigned to a billing address, it's not too big of a deal to use it to get service at another address, providing you don't try to report problems or have a service call dispatched.
So in summary, if you take your cable modem to a friend's house across town, who has the same ISP, it might work. Try it and see.
