# Address Resolution Protocol (ARP)

This protocol is used to update a nodes ARP cache within a network . When this node needs to send packet to a address within its network (as identified by the network mask ) but that entry is missing from the ARP cache it does the following.

• It broadcasts a frame with its IP address and MAC address as the source and the IP address and the all f’s address as the destination.
• All hosts on the network receive this frame .
  • Hosts that don’t have a matching IP address throw this frame away.
  • The host that matches this IP address first updates its ARP cache if it does not have the senders MAC address then responds with a unicast message.
• In receipt of this message the original node can now update its ARP cache and then complete sending its message.

If no host responds to the ARP message - the original message is dropped.