Hi,

I never had this problem. Perhaps it's a driver issue.

What network interface card do you use?

What do you see, when you open a terminal and enter
Code:
dmesg | grep eth
You could boot another linux live cd with a working wired network an try to find out which driver is used. Then you could look if this one is available on the Knoppix cd.

You could try the latest Knoppix 6.2.1.