MAC Address Clone
From DD-WRT Wiki
 MAC Address Cloning Entailments
This is a very useful option on your router, which is located in a sub tab under Basic Setup. Many ISP's require you to register your equipment's MAC (or Media Access Controller) address to be able to use their services, as this provides a method of keeping track of billing and/or bandwidth monitoring. The addresses are coded in hexadecimal format, and in this case are limited to the values of A-F and 0-9 restrictively.
To enable MAC Cloning, simply click the option under the tab itself. You will have the option of either "Get current PC MAC" or you can input one yourself. Get current PC MAC is primarily used when establishing a service with an ISP that requires it, such as Comcast. The other option of manually entering a MAC address can be used for many things such as linking two routers together, or "spoofing" to force your ISP to give you a new IP address.
"Spoofing" can be used if you would like to maintain anonymity on the internet, as once someone knows your IP, they can do a multitude of things such as hacking or banning you from your favorite website/game server(s). If you wish to use this option, make sure it does not violate your ISP's ToS, or "Terms of Service" agreement, and you must have an ISP that is dynamic . It is also not wise to do this if you have a Digital Phone service.