Many people report that Ubuntu Server suddenly reverts its static IP address to dynamic (via DHCP). Here are some discussions:
static ip reverts back to DHCP (ubuntuforums.org).
Static IP reverting to DHCP on Ubuntu Server 8.04 (www.howtogeek.com).
This happened to me today with Ubuntu Server 9.04. Rebooting the server after installation have solved this problem.
You need to change addresses into following files:
The following article from www.howtogeek.com prefers vi editor, but I use nano editor.
sudo nano /etc/network/interfaces
change to something like this
iface eth0 inet static
and DNS settings
sudo nano /etc/resolv.conf
and then restart
sudo /etc/init.d/networking restart
If you have DHCP client enabled, you must disable it. Consult the following article for more details:
Ubuntu DHCP to a Static IP Address