Normally your cablemodem should provide an ip adress to you.
In most cases the cablemodem is only a small router/gateway,in which a dhcp server is present which is programmed in this way to handover only 1 ip adress.
When using a cable modem the thing to watch out for when using 2 pc's ,and switching one pc to the other. Is that certain providers program their cablemodem in that way that the leasetime of the handed over ip adress could 1/2 hour and more.
This means that before trying the internet with another pc directly on the same cablemodem ,you first have to do a 'release' of your first pc.
In windoze you type in a command window : 'ipconfig /release' .
If you don't do that,you'll have to wait for 1/2 hour (or more) until you receive your ip adress from your cablemodem.(you'll notice then in knoppix when starting autoconfig for networkcard ,won't receive any ip).
hope this'll help .. Greeetzzz...