>At home, I have a cable connection. I never had any problem to hook the workstation to the net. However, I tried to do the same with several laptops and I never succeeded. My configuration on the workstation and all the laptops I have tried is the same. In the TCPIP panel, I only check Obtain an IP address automatically. Detect connection to network media is checked. I also entered my two DNS. That's it. I don't have anything else. However, the same doesn't work on the laptops. Whenever I run WINIPCFG, I can see that the subnet mask is not 255.255.255.0 but 255.255.0.0. The default gateway is empty the the IP address is not the same range as the workstation. There is something I can't see here but I guess it's pretty simple.
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>To clarify, I only use one connection. When I tried on the laptop, I simply remove the Internet cable from the workstation and plug it in the laptop. The cable modem doesn't change. All the lights are still effective. Is there some kind of security somewhere which doesn't allow anything else other than my workstation to be connected to that cable modem?

Reset your modem. It seems that the physical address of the network card is saved into the modem. Resetting it re-establish everything.