LAN Address Resolution Protocol (ARP)

• ARP Table: Each IP node (host or router) on a LAN has an ARP table that stores IP/MAC address mappings:
  • <IP address; MAC address; TTL>
  • TTL (Time To Live): The time after which the address mapping is forgotten (usually 20 minutes)

ARP Protocol in Action

• Scenario: Host A wants to send a datagram to Host B

  • B’s MAC address is not in A’s ARP table, so A uses ARP to find B’s MAC address

• ARP Query:

  • A broadcasts an ARP query with B’s IP address
  • Destination MAC address = FF-FF-FF-FF-FF-FF (broadcast address)
  • All nodes on the LAN receive the ARP query
  • A sends:
    • Source MAC: 71-65-F7-2B-08-53
    • Source IP: 137.196.7.23
    • Target IP address: 137.196.7.14

• ARP Response:

  • B replies with its MAC address, which is then stored in A’s ARP table