MAC ( Media Access Control )

A MAC (media access control) address is a unique numeric code that is permanently assigned to each unit of most types of networking hardware, such as network interface cards (NICs), by the manufacturer at the factory.

An NIC, also referred to as a network adapter, is a circuit board that is plugged into a slot on a motherboard (the main circuit board on a computer) to enable a computer to physically connect to a network cable and thereby communicate over a network (i.e., to one or more other computers). Some computers use network interface adapter circuitry that is built directly into the motherboard instead of a separate card.

The purpose of MAC addresses is to provide a unique hardware address or physical address for every node on a local area network (LAN) or other network. A node is a point at which a computer or other device (e.g., a printer or router) is connected to the network.

The code is most commonly a 48-bit hexadecimal (i.e., base 16) number, which consists of 12 characters. They are arranged in six pairs, each separated by a colon. A typical MAC will look something like 00:10:B5:C4:99:6A. The first 24 bits (three bytes) identify the manufacturer, and the remaining bits uniquely identify the type of device and provide a specific serial number for the unit.
When a computer is connected to a network, a correspondence table relates the computer's IP address to its physical address on the network. The MAC addresses of the sending computers are contained in the header of each packet, thus allowing packets to arrive at their intended destination.

An IP address is an identifier for a computer or other device on most networks, including the Internet. Every message sent over such networks is divided into packets prior to transmission and and then reassembled into the original message at the destination. The header also contains the IP addresses of the sender and receiver along with other information needed to move the packets from the source to the destination and reassemble them.

Although MAC addresses are generally described as being permanent, it is possible for users to change them.