Many ISPs (Internet Service Providers) tie the customer's IP address to the MAC address of the computer (or router) connected to the modem. The Physical Layer (Layer 1) is always the first layer that you should check when trouble-shooting a network. This layer includes, modems, routers, computers, cables, etc. So in order to be able to more thoroughly test your network, being able to swap out your current device connected to your modem is key. This can obviously be very tricky when the ISP has locked the IP address to a single MAC address.
Here's where spoofing your MAC address comes extremely handy, and doing it on Mac OS X Tiger, (unfortunately this command does not seem to work in Leopard) and I'm assuming most Linux/UNIX system, is by far the easiest I've ever seen.
1.) Simply open Terminal and type in the following command:
sudo ifconfig en0 lladdr aa:bb:cc:dd:ee:ff
(where aa:bb:cc:dd:ee:ff is the new desired MAC address)
After entering that command, Terminal will ask for your password and now all connected devices connected to the wired ethernet connection should see this new MAC address. And as if this tutorial couldn't get any easier, to revert, simply restart your Mac.