MAC/LAN/physical/Ethernet Address

• Used "locally" to get frame from one interface to another, physically connected interface (same network)
• 48 bit address burned into NIC ROM
• Written in hex
• Each adapter on a LAN has a unique address
• Allocation administered by IEEE
• Manufacturer buys a portion of MAC address space
• MAC flat address -> portability
• Can move between LANs
  • IP addresses aren't portable, depends on IP subnet

ARP example

• A wants to send datagram to B
• B's MAC isn't in A's ARP table
• A broadcasts ARP query packet, w/ B's IP address
• ARP query contains sender/receiver IP/MAC, with dest MAC as FF-FF-FF-FF-FF-FF
• B receives ARP packet
  • Responds only to A w/ ARP response packet, holding B's MAC
• A caches IP/MAC address pair in ARP table until information times out
• ARP is plug-n-play, no need for net admin

Switches