IMHO, I think that the problem is not with your cable modem but with the configuration of your network adaptor (a.k.a. the network card that the cable modem is plugged into).
If your network adapter is installed correctly, you have to enable DHCP at boot.
You should include more info in your post. Like maybe running ifconfig and route and showing the results.
There are also some good articles about setting up DHCP, like the DHCP mini howto