A MAC address should be unique to the hardware ie your box.
So if you had a MAG250 for instance the MAC address is fixed.
However you are using an emulator on an android box, in order to get your android box to work like a MAG250, so the program uses a virtual MAC address.
Your server uses the MAC address to give you access, so if you give him some details
and they have changed you can manually edit the MAC address in the emulator and save it.
If the MAC address on the emulator doesn't match the MAC address you give to your server it won't connect.