I use Windows 2000, and I’m able to release the ISP (Verizon) lease using the WNTIPCFG applet, but I cannot do the same when I use the Ubuntu v9.04 live CD. I have used just about all methods I found on the Internet, and none has worked, so please make sure your suggestion really works before answering. If the lease is truly released, a Windows connection on the same line will get a new lease immediately.
You are looking for the dhclient command, which is the DHCP client. Try this:
sudo dhclient -r
Sorry, I don't have enough status to comment. How do you know the address is not being released. You said the connection is broken off, but the address is not released.
Verizon may reassign an address based on MAC address, but if you release it and then try to renew it from Windows, the MAC on Windows is different and Verizon may give you an entirely different address.
Or, are you trying to get an entirely different address and you keep getting the same one whenever you renew? In that case, Verizon is probably reusing the last address released in which case you will need to leave your machine offline for maybe an hour or two before renewal will give you a new address.