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I am curious if I were to move, do you have to tell your ISP, in this case my cable company, that I am moving? Is the cable modem MAC restricted to a certain CMTS, or could I use any CMTS belonging to my ISP?

EDIT: I am interested in whether or not by virtue of me having internet service through a specific ISP, if my cable modem can connect to any CMTS my ISP has, or if I am restricted to just a one (or a subset). If I were to take my cable modem to a friend's house who has cable service through that same ISP (or no service at all, but at least having a cable into his house from that company), would it be able to get service at that location also using my cable modem?

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up vote 3 down vote accepted

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.

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Oh, it's more likely to work if the IP addresses in a traceroute from each location have the same IP addresses (save for any routers) in them, particularly the one after the round trip times go from < 10 ms to > 10 ms, which would be the cable router gateway. –  Nevin Williams May 23 '13 at 5:31
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If you own your cable modem, you can take it with you.

The only requirement with using the same cable modem on a new ISP is the protocol it supports.

For instance, my DOCSIS3 compatible Motorola Surfboard modem that I purchased and used in Chicago with Comcast is now powering my CableOne service in Oklahoma.

I didn't have to tell them it had been used before. I didn't have to unregister it or anything else.

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I don't think I worded my question well, so let me rephase. If I own my cable modem and I take it to my friend's house across town who has the same ISP, could I hook up my cable modem there successfully. The technical aspect of this that interests me is if the cable modem MAC is tied to a specific CMTS or not. –  Fred Thomsen Apr 15 '13 at 20:40
    
Edit your question to add clarification –  Canadian Luke Apr 15 '13 at 20:51
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It depends on the ISP. Comcast, for example, may require you to call them and register it, because they will only let the MAC address of your friend's current modem access their network.

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