MAC  = Media Access Control  ,  which  is a sublayer of the Data Link Layer that provides addressing and channel access control mechanisms that make it possible for several terminals or network nodes to communicate within a multipoint network, typically a local area network (LAN) or metropolitan area network (MAN).

The MAC address is a unique value associated with a network
adapter. MAC addresses are also known as hardware addresses or physical addresses. They uniquely identify an adapter on a LAN.

MAC addresses are globally unique addressed that are written into hardware at the time of manufacture. For this reason, MAC addresses are sometimes called Burned In Addresses (BIA).

The purpose of the MAC address is to uniquely identify every node on a network; nodes include workstations and devices like printers. Every adapter has a unique MAC address. Even two identical models from the same manufacturer will have distinct MAC addresses.

MAC addresses are 12-digit hexadecimal numbers ; 48 bits in length this means that there are 281,474,976,710,656 possible MAC addresses. By convention, MAC addresses are usually written in one of the following two formats:

MM:MM:MM:SS:SS:SS Or MM-MM-MM-SS-SS-SS

The first half of a MAC address contains the ID number of the adapter manufacturer , they are assigned by the IEEE  – Institute of Electrical and Electronics Engineers – and the database is available online at IEEE OUI and Company_id Assignments. The second half of a MAC address represents the serial number assigned to the adapter by the manufacturer. In the example,

00:A0:C9:14:C8:29

The prefix  " 00A0C9 " indicates the manufacturer is Intel Corporation , “   14C829 ” is the serial number.

MAC vs. IP Addressing

Whereas MAC addressing works at the data link layer, IP addressing functions at the network layer . It’s a slight
oversimplification, but one can think of IP addressing as supporting the software implementation and MAC addresses as supporting the hardware implementation of the network stack. The MAC address generally remains fixed and follows the network device, but the IP address changes as the network device moves from one network to another.

Finding your MAC address :

1- In Windows :

Open up your command prompt window by clicking Start, then Run, then
cmd. Once in the command prompt window, type ipconfig/all and the MAC
address will be visible

2-In  Linux :

First become the root using the su command. Then simply type the
command ifconfig -a. You should see a 12 digit address usually labeled
HWaddr .