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Troubleshooting Q&A - February 26, 2004

Cable Modem Not Working with Router

This week's Q&A diagnoses a common problem that can prevent your cable modem from properly connecting to and communicating with your router and networked PCs.

By Ron Pacchiano

Q. For the last few months I have been using a cable modem that works fine when connected to the Ethernet network adapter in my desktop PC. I recently purchased a new notebook computer that also needs access to the Internet. Unlike my desktop PC, I’d like the notebook to be capable of using the Internet from other locations around my house (i.e. bedroom, den, kitchen, deck, etc.), so I went to the local CompUSA and purchased a D-Link DI-614+ wireless router and a D-Link DEL-650+ PC Card for my notebook.

When I got home I installed the PC Card into my notebook and connected my cable modem to the router’s WAN port. For some reason, though, the router doesn’t appear to be working. Neither of the two PCs can get onto the Internet. I went into the router’s administration panel and checked the status of it, and it appears as though the router isn’t picking up an IP address from the cable modem. I thought that the problem might be with my cable modem, but when I reconnected it directly to my desktop PC, it worked fine.

I have tried a number of things to resolve this problem, but with no success. I even went as far as exchanging the router for another one, thinking that it might have been defective, but it wasn’t. This leads me to believe that I am doing something wrong; I just have no idea what that might be. Do you have any suggestions that might help me resolve this situation? I am at my wit’s end! Thank you!

A. You’ll be relieved to know that the solution to your problem isn’t as elusive as you might think. And unless I miss my guess, the answer is quiet simple. I believe that the cause of your problem has to do with the MAC address of your PC’s Ethernet adapter.

You see, when your cable modem is first initialized, it associates itself with the MAC address of the network adapter that it is connected to. In this case, that would be the network adapter in your desktop PC. When the cable modem is disconnected from the PC and then connected to the router’s WAN port, it can no longer locate the MAC address that it was originally associated with, preventing it from functioning.

This is a common characteristic of many cable modems and in most cases can easily be corrected. Before we begin, though, you should make a note of the IP address that your cable modem is assigning to your desktop PC when it’s directly connected to it. We’ll need that later to verify that the router is properly communicating with the cable modem.

Typically, all you need to do is unplug your cable modem from the power converter (or wall outlet) and leave it alone for a few minutes (5 to 10 minutes is usually sufficient). Once you’ve done this, reconnect it to the power outlet and wait a few minutes for the modem to be initialized (again, about 5 minutes).

Once the cable modem has been reinitialized, you can go ahead and connect it to your D-Link router’s WAN port. At this point, I would recommend you power cycle or reset your router. This will force the router to request a new IP address from the cable modem. Once this has been completed, your router should be online and communicating with the cable modem.

You can verify that the router is communicating with the cable modem by logging in to the router and clicking on the status tab. Under the WAN section you should see the same IP address (or at least a similar one) to the one you recorded earlier. If not, then your cable modem has not completed the initialization process, is still married to the old MAC address, or cannot connect to your service provider.

I should note that in some cases getting the modem to release a MAC address could take considerably longer than the 5 or 10 minutes I mention here. I know some people who have had to wait almost 45 minutes to get the router to reset itself properly, and others have needed to leave it unplugged overnight before it would reinitialize correctly. So don’t be discouraged if it doesn’t work immediately.

Now all you need to do is connect your workstations to the router. If you have DHCP enabled (which by default it is), you should find that all of your PCs have Internet access. If not, you have a problem with your network configuration, which you’ll need to examine separately.

Also, it’s possible that you’ve come across a modem that has become permanently linked to a specific MAC address. This may apply to you if you had to call your service provider to activate your service. In this situation you’ll need to have the router clone the MAC address that it is looking for. This capability is widely available in most routers and is enabled using the router’s administrative interface. For more information on cloning a MAC address, please refer to your router’s users manual.

Good luck!

Use our feedback form to submit your questions on home or SOHO networking issues. We cannot guarantee to answer every question we get, but we’ll consider them all. Earthweb HardwareCentral earthwebdeveloper CrossNodes Datamation

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