In very short:
Ubuntu Server 12.04.1
Changing the setup from DHCP to static, (/etc/network/interfaces).
iface lo inet loopback
# The primary network interface
iface eth0 inet static
dns-nameservers xxx.xxx.xxx.xxx, xxx.xxx.xxx.xxx (this is from your internet service provider) if you don't do this you wont be able to have internet acces.
By the way, you do not need to edit the /etc/resolv.conf file, It will be overwritten anyway at restart. And also don't need to uninstall the DHCP which a lot of people are mentioning in most forums. the dns-nameservers line will do the trick. (the ip are separated by e comma and a space)