Ok, when you install the auxillary NIC (so you have 2 NICs, the onboard and the extra), and you boot knoppix, and then go to the main console, and type 'ifconfig', what do you see?
Do you have the cable plugged into the add-in NIC?
Do you have a DHCP server available to answer DHCP requests?
do you see 1 or 2 'Ethernet controller' when you do 'cat /proc/pci'? (assuming your add-in NIC is PCI).
if you don't have a DHCP server handy, then 'ifconfig' will probablyl only show you lo0, and we can probably get your
NIC up, if it shows up in /proc/pci.
place the following in /etc/network/interfaces:
iface eth0 inet static
netmask 255.255.255.0
address 192.168.0.5
gateway 192.168.0.1
iface eth1 inet static
netmask 255.255.255.0
address 192.168.0.6
gateway 192.168.0.1
You might need to massage the IP addy's for your local subnet.
to use these, do 'ifup eth0' and then 'ifup eth1'.
Then you can see which of these is the true active ethernet controller.