Each MAC address is supposed to be globally unique. You can change the MAC address when configuring the interface and you can even write a new MAC address to the adapter’s EEPROM. But such alterations are generally only done for testing purposes and the only other excentric use of MAC adressing I know about is giving a Speedtouch Home ADSL modem the Ping of Life. This is because an important feature of the MAC address is that you were supposed to be able to rely on its uniqueness, and many programs assume they can. Mistaken they are ! A large supplier of ADSL modems once shipped us several batches of ADSL modems with variously duplicated MAC addresses. Worse : we accepted them. And we then had to find ourselves another unique identifier for the modems on our network… Just another invalid assumption…