If you’ve installed your Ubuntu installation with DHCP, it’s really easy to reconfigure your system to use a static IP address with the GUI based Network Settings dialog.
To open the utility, go to System \ Administration \ Networking
You’ll see your network adapter in the list. Click the Properties button, and you’ll see this dialog:
Just change the drop-down box to Static IP address, and then put in your static settings. Ubuntu is so easy, isn’t it ?