Address Resolution Protocol (ARP)

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• when two hosts communicate over an Ethernet network using IP,
  • IP address identifies each network layer host interface
  • but, transmission of data must take place at the Physical and Data Link layers
    • using MAC address of interface
  • TCP/IP suite includes the address resolution protocol (ARP) to resolve an IPv4 address to a hardware MAC address
• when both sending and receiving hosts are in the same broadcast domain or subnet:
  • local address resolution takes place using ARP requests and ARP replies

• if destination address is on a different subnet or remote network:
  • the local host must use its default gateway to forward the packet
    • must determine the MAC address of the default gateway using ARP
• router uses ARP messaging for its Ethernet interfaces
  • ARP messaging is only used with Ethernet
  • router public interface might use different type of framing and local addressing