The Ethernet Mac Address

The Ethernet MAC Address
Initially, Ethernet was implemented as part of a bus topology. Every network device was connected to the same, shared media. In low traffic or small networks, this was an acceptable deployment. The main problem to solve was how to identify each device. The signal could be sent to every device, but how would each device identify if it were the intended receiver of the message?
A unique identifier called a Media Access Control (MAC) address was created to assist in determining the source and destination address within an Ethernet network. Regardless of which variety of Ethernet was used, the naming convention provided a method for device identification at a lower level of the OSI model.
As you will recall, MAC addressing is added as part of a Layer 2 PDU. An Ethernet MAC address is a 48-bit binary value expressed as 12 hexadecimal digits.
















MAC Address Structure
The MAC address value is a direct result of IEEE-enforced rules for vendors to ensure globally unique addresses for each Ethernet device. The rules established by IEEE require any vendor that sells Ethernet devices to register with IEEE. The IEEE assigns the vendor a 3-byte code, called the Organizationally Unique Identifier (OUI).

IEEE requires a vendor to follow two simple rules:
• All MAC addresses assigned to a NIC or other Ethernet device must use that vendor's assigned OUI as the first 3 bytes.
• All MAC addresses with the same OUI must be assigned a unique value (vendor code or serial number) in the last 3 bytes.

The MAC address is often referred to as a burned-in address (BIA) because it is burned into ROM (Read-Only Memory) on the NIC. This means that the address is encoded into the ROM chip permanently - it cannot be changed by software.
However, when the computer starts up, the NIC copies the address into RAM. When examining frames, it is the address in RAM that is used as the source address to compare with the destination address. The MAC address is used by the NIC to determine if a message should be passed to the upper layers for processing.

Network Devices
When the source device is forwarding the message to an Ethernet network, the header information within the destination MAC address is attached. The source device sends the data through the network. Each NIC in the network views the information to see if the MAC address matches its physical address. If there is no match, the device discards the frame. When the frame reaches the destination where the MAC of the NIC matches the destination MAC of the frame, the NIC passes the frame up the OSI layers, where the decapsulation process take place.
All devices connected to an Ethernet LAN have MAC-addressed interfaces. Different hardware and software manufacturers might represent the MAC address in different hexadecimal formats. The address formats might be similar to 00-05-9A-3C-78-00, 00:05:9A:3C:78:00, or 0005.9A3C.7800. MAC addresses are assigned to workstations, servers, printers, switches, and routers - any device that must originate and/or receive data on the network.