Media Access Control (MAC) Address

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Each Ethernet network interface port has a unique hardware/physical address, called the media access control (MAC) address.

• aka ethernet address (EA) or extended unique identifier (EUI)
• aka local or physical address

Format

• Each frame of Ethernet data identifies the source MAC address and destination MAC address in fields in a header
• A MAC address consists of 48 binary digits (48-bit)
  • six bytes in size
• typically represented as 12 hexadecimal digits
  • notation of this differs based on system architecture
  • Hex is a numbering system often used to represent network addresses of different types
  • A hex digit can be one of sixteen values: 0–9 and then A, B, C, D, E, F
  • often displayed as:
    • six groups of two hexadecimal digits with colon or hyphen separators
      • E.g., 00:60:8c:12:3a:bc
    • no separators
      • 00608c123abc
    • three groups of four hex digits with period separators
      • 0060.8c12.3abc

Burned-in Addresses

• IEEE gives each network adapter manufacturer a range of numbers
  • hard coded into every interface produced with a unique id from their range
  • called burned-in address or universal address
  • first six digits (3 bytes or octets) identify the manufacturer
    • aka organizational unique identifier (OUI)
  • last six digits are a serial number
• organization can use locally administered addresses in place of the manufacturers’ universal coding system
  • can be used to make MACs meaningful in terms of location on the network
    • adds a significant amount of administrative overhead
  • defined by changing the universal/local (U/L) bit from 0 to 1
  • rest of the address is configured using the card driver or network management software
  • network admin’s responsibility to ensure all interfaces have a unique MAC

Broadcast Address

The I/G bit of a MAC address determines whether the frame is addressed to an individual node (0) or a group (1).

• 1 is used for broadcast and multicast transmissions
• MAC address of all 1s is the broadcast address ff:ff:ff:ff:ff:ff
• unicast transmission is one sent to an individual host
  • achieved by adding the host’s unique MAC address as the destination address
• when a frame uses the broadcast address as the destination address
  • should be processed by all nodes that receive the frame
  • are within the same broadcast domain