MAC address is also called the physical address, hardware address or link address, is written in the internal hardware production by network equipment manufacturers. IP address and MAC address in the computer are in binary notation, IP address is 32 bit, and the MAC address is the 48's. The length of the MAC address is 48 (6 bytes), usually expressed as 12 hexadecimal digits between each two hexadecimal digits separated by colons, such as: 08: 00: 20: 0A: 8C: 6D is a MAC address, of which the first six hexadecimal 08:00:20 representative network hardware manufacturer's number, which is assigned by the IEEE (Institute of Electrical and Electronics Engineers Institute of Electrical and Electronics Engineers), followed by three hexadecimal digits 0A: 8C: 6D on behalf of the manufacturer of a network of manufacturing products (such as network card) serial number. As long as you do not change your MAC address, then your MAC address is unique in the world.

0-23 bits are assigned by the manufacturers themselves. 24-47 bits, called Organizationally Unique Identifier (organizationally unique, is to identify the identity LAN (local area network) node. Among which, the 40th is a multicast address flag. NIC physical address is usually is burned into card EPROM (a flash memory chip by the card manufacturer, typically can be erased by the program), which stores the address of the transmission data on which the true identity of the host computer sends out the data and receiving data.

To summarize, MAC address, just as identity card numbers on our identity, globally unique.
Production of cards, routers, WIFI wireless products, Bluetooth products, manufacturers are required to apply for IEEE OUI code for the product, which is the manufacturer code of MAC addresses. After the factory has made this code, plus 24 (bite) of the manufacturers self device number, consisting of the device's MAC address.