Say you’ve got a Ubuntu 12.04 Precise Pangolin system with a DHCP address, and you want to release that address and obtain a new one (this is often useful in troubleshooting connectivity problems). Releasing a DHCP address in Ubuntu is quite simple. Go to a Terminal window and type this command:
sudo dhclient -r
(Note that you must use dhclient either as root or with the sudo command for it to work.)
This will release your current IP address. To obtain a new one, use the dhclient without any arguments:
Assuming that dhclient can reach a DHCP server, your system will receive a new IP address in short order.