MAC

The Media Access Control is often said to be a sub-layer of the OSI Data Link layer. On every network interface adaptor card, there is a set of computer chips that handle communication with the physical media (copper wire, fiber optic cable or the air) by controlling the communication signal (electricity, light or radio frequencies) over the physical media. In plain English, the computer chips that control the electricity transmitted and received on a copper wire are MAC-related hardware.

The MAC sublayer provides the means to access the physical medium used for communication. The MAC sublayer also communicates with the Logical Link Control (LLC) sub-layer above it allowing it to access and speak to the upper layer network protocols such as IP.

MAC Addresses:

Display Pattern	As Displayed by	Command(s) Used
0000.0C12.B1CF	Cisco	show interface sh int
0:1:5D:12:B1:CF	Unix/Oracle-Sun	ifconfig -a
	Red Hat Linux
001F28-12B1CF	HP ProCurve Switches	show bridge
00-03-FF-12-B1-CF	Microsoft Systems	ipconfig /all

Duplicate MAC Addresses:

The IEEE 802.3 Ethernet and 802.5 Token Ring protocols specify that the MAC sub-layer must supply a 48-bit (6 bytes) address. The MAC address is most frequently represented as 12 hexadecimal digits. The MAC address uniquely identifies a specific network device and MAC addresses must be unique on a given LAN (a network of computing devices in a single subnet of IP addresses). The first 12-bit portion of the MAC address identifies the vendor of the network device, the last 12-bit portion identifies the unique id of the device itself. When looking at a hexadecimal representation of the MAC address, the first six hexadecimal digits identify the vendor and the last six hexadecimal digits identify the specific network interface card.

Here are some examples of what a MAC address looks like. There is some difference in how they are displayed on different types of computers. The hexadecimal digits are the same, but they are separated or grouped differently when displayed. Different companies like to show MAC addresses different ways.

Manufacturers of network interface adaptor cards 'burn' a MAC address into the memory of the chips on every card they produce. The pattern of bits in the first set of 24 bits of the MAC address is assigned to a specific vendor as the Organizationally Unique Identifier (OUI). Cisco was assigned the hexadecimal prefix '00000C' to use on their first set of network interface adaptors.

In the case of the protocols specified in the IEEE's 802.x series of documents, the first 24 bits of a MAC address identify the vendor-manufacturer of the network interface card and the last 24 bits identify the card itself, or more precisely, the last 24 bits identify the specific host the network interface card is attached to. The 24 bits used to identify a host allows for up to 16.7 million unique card addresses on one network. Since there are more than 16.7 million computers in the world, this clearly isn't enough addresses for every computer on earth, is it?

Manufacturers re-use MAC addresses and they ship cards with duplicate addresses to different parts of the United States or the World so that there is only a very small chance two computers with network cards with the same MAC address will end up on the same network.

MAC addresses are 'burned' into the Network Interface Card (NIC), and cannot be changed. See ARP and RARP on how IP addresses are translated into MAC addresses and vice versa.In order for a network device to be able to communicate, the MAC address it is using must be unique. No other device on that local network subnet can use that MAC address. If two devices have the same MAC address (which occurs more often than network administrators would like), neither computer can communicate properly. On an Ethernet LAN, this will cause a high number of collisions. Duplicate MAC addresses on the same LAN are a problem. Duplicate MAC addresses separated by one or more routers is not a problem since the two devices won't see each other and will use the router to communicate.