Sometimes your ISP's store the MAC address of your ethernet card and associate it with the IP address they provide. Suppose you change your network card (or your machine), It will not be able to log on to the net because your ethernet card has a different MAC address than the one stored in the database of the ISP.
# ifconfig eth0 down
# ifconfig eth0 hw ether 00:80:48:BA:d1:20
# ifconfig eth0 up
# ifconfig eth0 |grep HWaddr
In that case you will have to call your ISP and ask them to reset the Mac address, or else you can change your MAC address to the one registered with your ISP :)