The Institute of IEEE (Electrical and Electronics Engineers ) has subdivided the Data Link layer into two sublayers called the LLC (Logical Link Control) and MAC ( Media Access Control). This section details the functions of MAC addresses and how they are used at layer 2 of the OSI Reference Model.

The MAC sublayer is used to create unique addresses used by Network- layer protocols to map the network address to the interface address so data can be routed to the interface. As mentioned earlier, MAC addresses are 48 bits in length and displayed as 12 hexadecimal digits. The first six hexadecimal digits are used to identify the manufacturer or vendor who produced the network interface. The second part of the MAC address is six hexadecimal digits composing a serial number assigned by the interface’s manufacturer or vendor.

Also explained above, MAC addresses are called burned-in addresses (BIAs), or hardware addresses, because they are burned into read-only memory (ROM). Different Network-layer protocol suites use unique methods to perform mappings from their addresses to the MAC address. For example, IP uses ARP (Address Resolution Protocol).

A device on the network that needs to send data to another network device must know where the device resides in order to send data to it. When the destination device resides on a remote network, the sending host sends an ARP request for the MAC address of its default gateway. The host receives a reply with the MAC address of the default gateway and then sends the data, including the IP address of the destination host, to the router. The router then forwards the data to the next hop based on information learned or manually inserted into the router’s routing table. This occurs for each hop the data takes through the networks needed to deliver the data packets to the network on which the destination host resides.

Once the destination router receives the data, it checks an ARP table to see if it knows the MAC address assigned to the host it received data for. If the router does not find an entry in its table for the IP address, it sends an ARP broadcast on the network to learn the MAC address for the receiving host. The device using the IP address listed in the ARP broadcast returns the message with a reply containing the MAC address the host is using.

Another way of mapping MAC addresses to the Network-layer addresses is the Hello Protocol. This Network-layer protocol allows hosts to identify themselves and indicate that they are still functioning on the network. When a new host joins the network, it sends a hello message advertising itself. The other hosts on the network each send hello replies containing their MAC addresses to indicate their existence on the network. At specific intervals, hello messages are also sent to all the devices on the network to notify other hosts on the network they are still on the network.

A third way of mapping ARP addresses is called predictable MAC addressing. There are three protocols that use predictable MAC addresses: Xerox Network Service (XNS), Novell Internetwork Packet Exchange (IPX), and DECnet Phase IV. These protocols embed the MAC address into the Network layer addresses they use on the network.

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