6.1 | Ethernet Fundamentals | |||
6.1.4 | Naming |
To allow for local delivery of frames on
the Ethernet, there must be an addressing system, a way of uniquely
identifying computers and interfaces.
Ethernet
uses MAC addresses that are 48 bits in length and expressed as twelve
hexadecimal digits. The first six hexadecimal digits, which are
administered by the IEEE, identify the manufacturer or vendor. This
portion of the MAC address is known as the Organizational Unique
Identifier (OUI). The remaining six hexadecimal digits represent the
interface serial number, or another value administered by the specific
equipment manufacturer.
MAC
addresses are sometimes referred to as burned-in addresses (BIA)
because they are burned into read-only memory (ROM) and are copied
into random-access memory (RAM) when the NIC initializes. At the data link layer MAC headers and trailers are added to upper layer data. The header and trailer contain control information intended for the data link layer in the destination system. Data from upper layer entities is encapsulated in the data link layer header and trailer. The NIC uses the MAC address to assess whether the message should be passed onto the upper layers of the OSI model. The NIC makes this assessment without using CPU processing time, enabling better communication times on an Ethernet network. On an Ethernet network, when one device sends data it can open a communication pathway to the other device by using the destination MAC address. The source device attaches a header with the MAC address of the intended destination and sends data onto the network. As this data propagates along the network media the NIC in each device on the network checks to see if the MAC address matches the physical destination address carried by the data frame. If there is no match, the NIC discards the data frame. When the data reaches the destination node, the NIC makes a copy and passes the frame up the OSI layers. On an Ethernet network, all nodes must examine the MAC header even if the communicating nodes are side by side. All devices that are connected to the Ethernet LAN have MAC addressed interfaces including workstations, printers, routers, and switches.
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