MAC address cloning
Hi killer - MAC (Media Access Control) cloning is used to clone your MAC address.
Some ISPs (most notably MediaOne) use your NIC's MAC address to authenticate your service. When you change the computer that you have connected to your cable modem service, you usually have to call up Tech Support and give them the MAC address of the NIC in the new machine. Since hardware routers have the equivalent of a NIC on their WAN port in order to connect to your cable modem, you would have to do the same thing if you're installing a router.
Recently, however, some ISPs have started to recognize the MAC addresses of popular routers and disconnect service if a router is detected. Fortunately, many router manufacturers are including the capability to either set the router WAN port MAC address to anything you want, or copy it from a computer connected to the router LAN side. You can check the Router comparison chart under "Mac addr. clone" to see which routers support this feature. Then consult the router's documentation for directions on using the feature.
So basically, it's used to fool ISPs into thinking there isn't a router. If the router has the same MAC address as either your NIC card or your modem the ISP won't detect and service won't be interrupted. Most ISPs don't do this so iy's rarely needed.
If this answers your question PLEASE rate this as fixed. If you need more help just add a comment and I'll be happy to assist you further. Thanks.