From cybercity.biz
Linux renew ip command using dhcp
Warning: Releasing your IP address always brings down your network interface (NIC) and WiFi. So be careful with remote systems. Do not run these commands over ssh command-based session.
The -r flag explicitly releases the current lease, and once the lease has been released, the client exits. For example, open terminal application and type the command:
sudo dhclient -rNow obtain fresh IP address using DHCP on Linux:
sudo dhclient