Can't connect to Wired Network after installing 12.04

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http://askubuntu.com – I have installed 12.04 into a used HP Compaq DC 7100 CMT. Plugged in LAN cable into the computer and it says network disconnected. from what I understand on the things I've read, it's not getting the address right, maybe it's just a guess (hardware address 00:16:35:78:47:bb). I know the LAN cable is working I can get on the net with the old computer. (Old computer is using device MAC address; Auto eth0 00:11:11:E6:4F:FE). I have put this address into the newer HP and it shows last used (minutes ago) but no Internet sever not found. I tried the hardware address it says never. I feel and thin (HowTos)