The IP addresses associated with any network cards you might have are read from the file /etc/network/interfaces.

A sample entry for a machine with a static address would look something like this:

# The loopback network interface

auto lo

iface lo inet loopback

# The primary network interface

auto eth0

iface eth0 inet static

address 192.168.3.90

gateway 192.168.3.1

netmask 255.255.255.0

network 192.168.3.0

broadcast 192.168.3.255

Dhcp

For a machine running DHCP the setup would look much simpler:

# The loopback network interface

auto lo

iface lo inet loopback

# The primary network interface - use DHCP to find our address

auto eth0

iface eth0 inet dhcp

changes to this file you can cause them to take effect by running:

/etc/init.d/networking restart