Ref:  Cisco Documentation CD June 2000, http://127.0.0.1:8080/cc/td/doc/cisintwk/ito_doc/introint.htm#xtocid193923
 

Figure 1-14: MAC addresses, data-link addresses, and the IEEE sublayers of the data-link layer are all related.

MAC addresses are 48 bits in length and are expressed as 12 hexadecimal digits. The first 6 hexadecimal digits, which are administered by the IEEE, identify the manufacturer or vendor and thus comprise the Organizational Unique Identifier (OUI). The last 6 hexadecimal digits comprise the interface serial number
or another value administered by the specific vendor. MAC addresses sometimes are called burned-in addresses (BIAs) because they are burned into
read-only memory (ROM) and are copied into random-access memory (RAM) when the interface card initializes. Figure 1-15 illustrates the MAC address
format.
 

Figure 1-15: The MAC address contains a unique format of hexadecimal digits.

Different protocol suites use different methods for determining the MAC address of a device. The following three methods are used most often
Address Resolution Protocol (ARP) maps network addresses to MAC addresses.
Hello protocol enables network devices to learn the MAC addresses of othernetwork devices. MAC addresses are either embedded in the network-layer address or are generated by an algorithm.

Address resolution is the process of mapping network addresses to Media Access Control (MAC) addresses. This process is accomplished by using the
ARP, which is implemented by many protocol suites.When a network address is successfully associated with a MAC address, the network device stores the
information in the ARP cache. The ARP cache enables devices to send traffic to a destination without creating ARP traffic because the MAC address of the
destination is already known.

The process of address resolution differs slightly, depending on the network environment.
Address resolution on a single LAN begins when End System A broadcasts an ARP request onto the LAN in an attempt to learn the MAC address of End System B. The broadcast is received and processed by all devices on the LAN, although only End System B replies to the ARP request
by sending an ARP reply containing its MAC address to End System A. End System A
receives the reply and saves the MAC address of End System B in its ARP cache. (The ARP cache is where network addresses are associated with MAC
addresses.)Whenever End System A must communicate with End System B, it checks the ARP cache, finds the MAC address of System B, and sends the
frame directly without first having to use an ARP request.

Address resolution works differently, however, when source and destination devices are attached to different LANs that are interconnected by a router. End System Y broadcasts an ARP request onto the LAN in an attempt to learn the MAC address of End System Z. The broadcast is received and processed by all devices on the LAN, including Router X, which acts as a proxy for End System Z by checking its routing table to determine that End System Z is located on a different LAN. Router X then replies to the ARP request from End System Y, sending an ARP reply containing its own MAC address
as if it belonged to End System Z. End System Y receives the ARP reply and saves the MAC address of Router X in its ARP cache in the entry for End System Z. When End System Y must communicate with End System Z, it checks the ARP cache, finds the MAC address of Router X, and sends the frame directly without
using ARP requests. Router X receives the traffic from End System Y and forwards it to End System Z on the other LAN.