It would help if you would post the results of
ifconfig eth0
(or whatever your interface is).
In any case, you say you have its IP address, which would no doubt have been allocated by your router or cable modem as the case might be unless you set it up manually - you may need it (see below).

Is MTU less than 1500 (e.g.64) ?
Then enter
ifconfig eth0 mtu 1500

and/or

you might have to invoke netcardconfig, respond NO to DHCP question, and enter your IP address as found above.

BTW this might be applicable: http://www.knoppix.net/forum/viewtopic.php?t=26867