Here is a link for Ubuntu Server Network Configuration. The guide is valid for 12.04 version.
https://help.ubuntu.com/12.04/serverguide/network-configuration.html
Below are some important parts:
Static IP Address Assignment
To configure your system to use a static IP address assignment, add the static method to the inet address family statement for the appropriate interface in the file /etc/network/interfaces. The example below assumes you are configuring your first Ethernet interface identified as eth0. Change the address, netmask, and gateway values to meet the requirements of your network.
DNS Client Configuration
Traditionally, the file /etc/resolv.conf was a static configuration file that rarely needed to be changed or automatically changed via DCHP client hooks. Nowadays, a computer can switch from one network to another quite often and the resolvconf framework is now being used to track these changes and update the resolver’s configuration automatically. It acts as an intermediary between programs that supply nameserver information and applications that need nameserver information. Resolvconf gets populated with information by a set of hook scripts related to network interface configuration. The most notable difference for the user is that any change manually done to /etc/resolv.conf will be lost as it gets overwritten each time something triggers resolvconf. Instead, resolvconf uses DHCP client hooks, and /etc/network/interfaces to generate a list of nameservers and domains to put in /etc/resolv.conf, which is now a symlink:
/etc/resolv.conf -> ../run/resolvconf/resolv.conf
To configure the resolver, add the IP addresses of the nameservers that are appropriate for your network in the file /etc/network/interfaces. You can also add an optional DNS suffix search-lists to match your network domain names. For each other valid resolv.conf configuration option, you can include, in the stanza, one line beginning with that option name with a dns- prefix. The resulting file might look like the following:
auto eth0 iface eth0 inet static address 10.0.0.100 netmask 255.255.255.0 gateway 10.0.0.1 dns-search example.com dns-nameservers 192.168.3.45 192.168.8.10