Cable Modem Troubleshooting Tips


Swapping computers on the cable modem

Most domestic cable ISPs (including NTL, Blueyonder) have the cable modem configured to recognise only one client PC, by the MAC address of its network interface. Once the cable modem has learnt the MAC address of the first PC that talks to it, it will not respond to another MAC address in any way. Thus if you swap one PC for another (or for a router), the new PC (or router) will not work with the cable modem, because the new PC (or router) has a different MAC address to the old one. To reset the cable modem so that it will recognise the new PC, you must power the cable modem off and on again. Once the cable modem has rebooted and gone fully online again (indicator lights settled down), reboot the newly connected PC so that it makes a DHCP request, or manually make it request a new DHCP lease.

With some ISPs, it appears that, although resetting (or power cycling) the cable modem is enough to clear the table of learnt MAC addresses in the cable modem, it is not enough to force the ISP's DHCP server to lease an IP address to the newly connected PC. It is suggested that, before the first PC is disconnected from the cable modem, it should explicitly release its DHCP lease.

If even that is not enough, it might be necessary to wait (with the cable modem powered off) for the expiry time of the original DHCP lease (this is no longer necessary on the NTL network).

With NTL stand-alone cable modems, a simple cable modem power off and on again should permit a swap of connected PC, but there is a limit on the number of MAC address changes which are permitted within one 4-hour period (irrespective of DHCP lease time). No more than 3 different MAC addresses can be served within one 4-hour period, and only one at a time. After having used 3 different MAC addresses within the last 4 hours, if you want to connect a 4th different MAC address, you might have to wait up to 4 hours with the cable modem powered off.

With NTL digital TV set top boxes, if you change the connected device, then after the power off and on again, you need to register the new client MAC: see First connection to Pace set top box.

With the special Xbox Live service on NTL, the cable modem is programmed to provide service to a maximum of two (rather than one) customer MAC addresses simultaneously. Apart from this, the same principles listed above apply if you wish to swap the connected device(s).

With some cable ISPs, including Blueyonder, service will be provided only to known user MAC addresses, which have to be registered in advance with the ISP. This makes swapping to new PCs, or inserting a new NAT router, difficult or tedious. One workaround is to clone a previously registered MAC address into the new network interface card, or NAT router. For instance, most NAT routers provide configuration options for cloning the MAC address of the WAN port. When you swap devices that have MAC addresses cloned to be the same, then neither the cable modem nor the ISP's DHCP system will notice the change. The new device will need to issue a DHCP request.

If both old and new PCs are connected by USB to the cable modem, then these problems do not arise, because the USB driver software emulates an ethernet MAC address, and that emulated MAC address will be the same in both cases, so the cable modem does not notice the change. All that is necessary is to release the DHCP lease on the old PC, change the USB cable to connect the new PC to the cable modem, and request a DHCP lease on the new PC.

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