Although the physical MAC address is permanent by design and is assigned to your network card by factory several mechanisms allow the modification, or spoofing of the MAC address that is reported by the operating system.
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This can be useful if you want to keep your privacy or to ensure interoperability. Some internet service providers bind their service to a specific MAC address; if you change your network card or intend to install a router the service wont work anymore. Changing the MAC address of the new interface will solve the problem.