You don't say, but what is the issue of Knoppix that you are using, and have you tried the network card configuration command under the Knoppix menu ?
Try accepting DHCP mode first.
If that doesn't work, look at the output of
Code:
ifconfig
; assuming that your network card has been configured as e.g. eth0 , then make a note of the IP Address that should have been allocated by the router, also the router gateway/nameserver address (or get them from your Ubuntu system - you should find this in the /etc/resolv.conf file). Then try the network configuration again, this time entering the data manually instead of accepting DHCP. NB DON'T use 255.255.255.255 for the broadcast, try X.Y.Z.255 where X.Y.Z. is your local subnet (likely to be something like 192.168.2. as in the case with my Belkin router).

If it still doesn't work
1] Search the forum - these issues have all been posted before
2] Post the output of
Code:
ifconfig
3] Install proper Debian instead [EDIT:] (that is, if you are referring to an installed system. Otherwise, Knoppix 5.1.1 is getting a bit old as live CDs go, and may not support very recent hardware)