The first 3 octets of the MAC address are assigned by the IEEE OUI Registration Authority upon request by manufacturer of the device (either WiFi or Ethernet). Once the manufacturer has owned the OUI, it can then assign a unique MAC address (concatenating OUI with the manufacturer’s assigned lower 3 octets) for every device it creates (around 2[math]^24 = [/math]16777216 MACs).

The MAC address is tied to the hardware and usually buried in the ROM/EEPROM (for example these MAC-chips). Some other devices use virtual MAC addresses (the last 3 octets are changeable) and possible to cause MAC address conflict.