MAC Addresses

• 32-bit IP address:
  • Network-layer address for an interface
  • Used for layer 3 (network layer) forwarding
  • Example: 128.119.40.136
• MAC address (also called LAN, physical, or Ethernet address):
  • Function: Used locally to transfer a frame from one interface to another within the same subnet (in terms of IP addressing)
  • 48-bit MAC address:
    • Burned into NIC ROM (Network Interface Card Read-Only Memory)
    • Sometimes software-settable
  • Example: 1A-2F-BB-76-09-AD
• Hexadecimal (base 16) notation:
  • Each numeral represents 4 bits
• Router sends data through LAN Address Resolution Protocol (ARP)

Interface Addresses on a LAN

• Each interface on a LAN has:
  • A unique 48-bit MAC address
  • A locally unique 32-bit IP address

MAC Address Administration

• MAC address allocation is administered by IEEE
• Manufacturers buy a portion of MAC address space to ensure uniqueness
• Analogy:
  • MAC address: like a Social Security Number
  • IP address: like a postal address

MAC Address Portability

• MAC address flat: Portable; can move an interface from one LAN to another
• IP address: Not portable; depends on the IP subnet to which the node is attached