Here is a howto about the configuring ip using networking related files in Ubuntu / Debian systems.
The interfaces file contains the details of all configurations...

File: /etc/network/interfaces
This file contains network interface configuration information for the ifup and ifdown commands. This is where you configure how your system is connected to the network.
Three important Interfaces are:
# lo: Loopback interface (it is an internal
networking mechanism. It is used to
test applications...)
# eth0: First ethernet interface card
# wlan0: First wireless network interface