What is a MAC Address?

Whether you work in a wired network office or a wireless one, one thing is common for both environments: It takes both network software and hardware (cables, routers, etc.) to transfer data from your computer to another—or from a computer thousands of miles away to yours.
And in the end, to get the data you want right to YOU, it comes down to addresses.
So not surprisingly, along with an IP address (which is networks software), there's also a hardware address. Typically it is tied to a key connection device in your computer called the network interface card, or NIC. The NIC is essentially a computer circuit card that makes it possible for your computer to connect to a network.

An NIC turns data into an electrical signal that can be transmitted over the network.

Hey Nick. Meet Mac.

Every NIC has a hardware address that's known as a MAC, for Media Access Control. Where IP addresses are associated with TCP/IP (networking software), MAC addresses are linked to the hardware of network adapters.
A MAC address is given to a network adapter when it is manufactured. It is hardwired or hard-coded onto your computer's network interface card (NIC) and is unique to it. Something called the ARP (Address Resolution Protocol) translates an IP address into a MAC address. The ARP is like a passport that takes data from an IP address through an actual piece of computer hardware.
Once again, that's hardware and software working together, IP addresses and MAC addresses working together.

For this reason, the MAC address is sometimes referred to as a networking hardware address, the burned-in address (BIA), or the physical address. Here's an example of a MAC address for an Ethernet NIC: 00:0a:95:9d:68:16.

As you've probably noticed, the MAC address itself doesn't look anything like an IP address (see yours here). The MAC address is a string of usually six sets of two-digits or characters, separated by colons.
Some well-known manufacturers of network adapters or NICs are Dell, Belkin, Nortel and Cisco. These manufacturers all place a special number sequence (called the Organizationally Unique Identifier or OUI) in the MAC address that identifies them as the manufacturer. The OUI is typically right at the front of the address.
For example, consider a network adapter with the MAC address "00-14-22-01-23-45." The OUI for the manufacture of this router is the first three octets—"00-14-22." Here are the OUI for other some well-known manufacturers.
Dell: 00-14-22
Nortel: 00-04-DC
Cisco: 00-40-96
Belkin: 00-30-BD
It's common for the larger manufacturers of networking equipment to have more than one set of OUIs.

Networks and MAC addresses.

All devices on the same network subnet have different MAC addresses. MAC addresses are very useful in diagnosing network issues, such as problems with IP addresses.
MAC addresses are useful for network diagnosis because they never change, as opposed to a dynamic IP address, which can change from time to time. For a network administrator, that makes a MAC address a more reliable way to identify senders and receivers of data on the network.

Wireless Routers and MAC Filtering

On wireless networks, a process called MAC filtering is a security measure to prevent unwanted network access by hackers and intruders. In MAC address filtering, the router is configured to accept traffic only from specific MAC addresses. This way, computers whose MAC addresses are approved will be able to communicate through the network—even if they were given a new IP address by DHCP.
Meanwhile, a hacker who's hijacked a network IP address will be blocked because their MAC address will not be on the approved list and will be filtered out.