By default recent Ubuntus (e.g., 11.10) use Network Manager, which makes things simple:
- click network icon in system tray
- Edit Connections
- select the one entry that you see there (unless your network is not working)
- Edit…
- IPv4 Settings
- From Method drop-down select Manual
- the rest should be straight forward
If not using Network Manager to manage network connections, you need to edit /etc/network/interfaces and add (adjust to own network parameters):
auto eth0
iface eth0 inet static
address 192.168.0.100
netmask 255.255.255.0
network 192.168.0.0
broadcast 192.168.0.255
gateway 192.168.0.1