MAC, Media Access Control, address is a globally unique identifier assigned to network devices, and therefore it is often referred to as hardware or physical address. MAC addresses are 6-byte (48-bits) in length, and are written in MM:MM:MM:SS:SS:SS format. The first 3-bytes are ID number of the manufacturer, which is assigned by an Internet standards body. The second 3-bytes are serial number assigned by the manufacturer.
MAC layer represents layer 2 of the TCP/IP (adopted from OSI Reference Model), where IP represents layer 3. MAC address can be thought of as supporting hardware implementation whereas IP address supports software implementation. MAC addresses are permanently burned into hardware by hardware manufacturer, but IP addresses are assigned to the network devices by a network administrator. DHCP relies on MAC address to assign IP addresses to network devices.
How do I find a MAC address of network device?
Operating Systems support various command-line and GUI utilities to allow users to find MAC address of the system. On Unix variants including Solaris and Linux support "ifconfig -a", "ip link list" or "ip address show" command that displays MAC address of the network device among other useful information. Windows including NT, 2000, XP and 2003 support "ipconfig /all" command that displays MAC address. On a MacOS, one can find MAC address by opening "System Preferences", then selecting "Network".