MAC Address is a unique identifier of 48 bits assigned to network interfaces.

This Address is unique in the world, so it does identify you, everywhere.

From a Security & Privacy perspective this is not good at all!!!

How to modify it:

1. Open Terminal app.
2. Type:
   sudo ifconfig [en0-en1] ether [MAC Address]
   • where you can choose en0 for wired ethernet interface or en1 for airport interface
   • MAC Address is something like FF:FF:FF:FF:FF:FF
     where there are 6 groups of 2 exadecimal symbols separated by colons (:)
3. Insert your Administrator password.
4. Go to Network System Preferences, select active interface and push Advanced… button.
5. In TCP/IP tab click on Renew DHCP Lease button.
6. You should now see your new MAC Address in Hardware tab.

NOTE: you can set Configure: Manually in Hardware tab to manually set MAC Address and avoid using Terminal.

NOTE2: if you do not renew DHCP Lease you may loose network connectivity.

NOTE3: if you have set MAC Address through Terminal it will be restored to it own default after reboot or shutdown.

NOTE4: some Hotels or private Acces Point use MAC Filtering to prevent abuse of their network. The changing of MAC Address allow you circumviate this kind of restrictions (see MAC Spoofing). This is NOT legitimate, so if you are not an evil user and you need emergency network access, try to set your MAC Address to FF:FF:FF:FF:FF:FF (could not work of course). This way System Administrator will readily understand what is going on. 😉