[ubuntu] problem with cable modem+usb

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http://ubuntuforums.org – I'm trying to help a friend that has some problem connecting to the Internet. the problem is like that :the eth0 is ok, when used with adsl modem and dialer connected without a problem. now he switched to cable modem which should give him ip. ifconfig shows nothing about ipv4 address. he switched to a non dialer connection (should get ip via dhcp automatically) just in case, but nothing. i suggested he would try connect a usb instead of Ethernet and for a while it was ok, somehow. now again, no INTERNET, no ip. what can i check to see why his computer don't pull any ip via cable+usb? and idea would be greatly appreciated. it's ubuntu 9.10. (HowTos)