The MAC address is the network address of an interface at data link layer in IEEE 802 networks such as ethernet.
The ]media access control (MAC) address](http://en.wikipedia.org/wiki/MAC_address) is the network address of an interface at link layer in IEEE 802 networks such as ethernet. It is supposed to be globally unique, that is to say, there are no two network interfaces with the same MAC address. It is denoted by six octets written in hexadecimal and delimited by colons, like 00:33:66:99:CC:FF; sometimes hyphens are used as delimiters, like 00-33-66-99-CC-FF. The left three octets form the Organizationally Unique Idenitifier (OUI), which identifies the organization that issued the MAC address.
Link-layer address such as MAC addresses are also known as physical addresses or hardware addresses. On a link MAC addresses must be unique. For transmission on a link the link-layer address (e.g. the MAC address) matters. Therefore, logical addresses of the network layer like IP addresses must be translated to link-layer addresses. (If the target node is on-link, then the node transmits directly to the target, if the target node is off-link, then the node transmits via an on-link router.) For the translation from IPv4 to MAC the Address Resolution Protocol (ARP) is used, for the translation from ipv6 to MAC the Neighbor Discovery Protocol is used, which is part of ICMPv6.
