Cable Modem Troubleshooting Tips


Registration issues

Cable ISPs normally require one or more of the following things before service can be provided to your cable modem:

On this page:

Modem MAC pre-registration

If your cable ISP requires you to let them know the HFC MAC address of your cable modem, see Finding the HFC MAC Address of the cable modem, then communicate it to your ISP as they require.

NTL note: As of June 2001, NTL are operating an automated cable modem registration procedure that eliminates the need to communicate the HFC MAC address of the cable modem to them before the cable modem will work. Please follow whatever instructions NTL sent you in their Welcome letter. You will not need to know your HFC MAC address for the online registration procedure, but you might need to know it when requesting technical support.

As from mid-April 2001, unregistered cable modems are automatically given a limited cable modem service: the cable modem has an configuration file, with only limited bandwidths, limited on-line time, and no ability to browse anywhere other than NTL's auto-registration site. They will be given IP addresses in the range to These customers do not get full service until they have completed their registration online. This auto-recognition procedure works only for cable modems having HFC MAC addresses identifying them as one of the supported models on the NTL network:

In NTL ex-C&W franchise areas, cable modem service is provided through the Pace digital TV set-top-boxes. These STBs are allocated a configuration file until you request and are granted broadband service. STB users (unlike stand-alone cable modem users) have to register the MAC address of the network interface of the device connected to the STB before that device can be used with the cable connection. They may register up to five different MAC addresses, but can use only one at a time (see Swapping PCs on the cable modem).

The registration process is completed after you are on-line with your cable modem, using a web browser as described below in Post-Installation Registration.

PC network interface MAC pre-registration

If your ISP requires you to tell them the MAC address of your PC's network interface, then see Finding the MAC address of a PC's network interface, then communicate it to your ISP as they require. If you use a router, you should tell them the MAC address of the router's WAN port instead.

NTL users receiving broadband service through a digital TV set-top box, please see First connection to Pace set top box on NTL below.

Blueyonder users have to register the MAC address of the network interface of the device connected to the cable modem before that device can be used with the cable connection. They may register up to five different MAC addresses, but can use only one at a time (see Swapping PCs on the cable modem below). MAC registration can be done online (via any connection, not just a cable one) through the Self Care web site:, or, if that is not working, by telephone to Tech Support on 0800 953 2000. If you are using a NAT router connected to the cable modem, you need to register only the MAC address of the WAN port of the router, and not the MAC addresses of each PC connected to the LAN ports of the router.

First connection to set top box on NTL

The first time each PC (or router) connects to the cable modem port of a digital TV set top box (STB) on the NTL cable network, then (having re-booted the STB) you should launch a web browser (which must have JavaScript enabled) and attempt to connect to any simple web site (one which does not have any "/" characters in its URL). You will be redirected to a special registration site for customer PC MAC addresses: if not, try sending the browser to start.ntl. Follow the instructions on the display, and enter a unique name by which this PC will be known to this STB.

It is necessary to repeat this procedure if you connect to the STB another PC (or router) which has never previously been registered with this STB. Once a PC (or router) has been registered with the STB, you only need to re-boot the STB when swapping to it.

If the device you have just connected to the STB is a router, then you should perform the registration procedure via a web browser in one of the LAN clients of the router, although it will be the MAC address of the WAN port of the router that you will be registering, not that of the PC running the browser. When you have completed the MAC registration page (or you are asked to close the browser and restart), restart the router (or make it request a new DHCP lease), not the PC.

This process of registering the client MAC address is quite separate from the registration of the cable modem HFC MAC address with the ISP.

Note that the MAC registration screen is only available to devices whose MAC address has not been previously registered. Once the MAC address has been registered, it is never possible to get back to the MAC registration screen. The site start.ntl does not actually exist.

Post-installation NTL registration

You must have received an ntl:home Welcome letter containing:

These enable you to register online for ntlworld ISP services such as e-mail and web space. Cable modem users are not permitted access to the internet (apart from the registration server) until this registration has been completed.

Registration within the Broadband Installer CD-ROM

Normally, the registration for ntlworld ISP services is performed within the Broadband Installation procedure on the CD-ROM. The registration procedure starts with quoting a serial number:

You are then prompted for the other data from your Welcome letter. If at this stage, the registration fails to complete (e.g. because of failures within the network or registration servers), you need to return later to complete the registration manually, as below. In the meantime, you will not be able to access any part of the internet apart from the registration server URL given below. If it is inaccessible, keep trying until it is working.

Manual Auto-Registration procedure

In order to complete the registration of your cable modem or set top box, and to register for ntlworld username and ISP services, make sure that your PC is using your cable modem connection (not dial-up), then launch your web browser, which must have JavaScript enabled. If your web-browser hangs or gives an error page when launched, this might be because it is trying to connect to a home page which is disallowed until you have completed this registration: just click Stop. Send your web browser to (note the https rather than http) and follow the on-screen instructions. When asked for a Serial Number:

There will be a long pause while the registration system verifies the identity of your cable modem: if this never finishes, this might be because your browser does not have JavaScript enabled. You are then asked for other data from your Welcome letter (be sure to maintain the correct upper-case and lower-case distinction). Then follow the on-screen instructions.

After successful online registration, it is then necessary to power the cable modem off and on again (do not mistake the SB4100 front standby button for a power button) to load the proper configuration file and obtain the intended class of service, and then re-boot your PC. You will know when you get a final configuration when your PC gets given an IP address outside the temporary ranges. Surfboard users can check the class of service they have obtained by viewing the cable modem log after the re-boot.

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