Type the following command from the terminal prompt. If you are using a distro other than Ubuntu, make sure that your user account has sudo permissions, otherwise you might have to use su to switch to root before entering the command.
sudo dhclient -r
Internet Systems Consortium DHCP Client V3.0.5 Copyright 2004-2006 Internet Systems Consortium. All rights reserved. For info, please visit http://www.isc.org/sw/dhcp/ Listening on LPF/eth0/00:0c:29:3d:6e:71 Sending on LPF/eth0/00:0c:29:3d:6e:71 Sending on Socket/fallback
You can restart networking if you are having problems by using the following command:
sudo /etc/init.d/networking restart