Address Resolution Protocol (ARP)

ARP resolves IP Addresses to hardware MAC Addresses.

ARP Request for Local Network

• IP determines that the address it wants to send to is on the local network
• Source host checks its own list (ARP cache) for the MAC of the destination host
• If no match is found, ARP builds a request which includes its own IP and MAC and broadcasts for the IP and MAC address of the destination host
• Every host on the local network responds to the broadcast by checking if the IP address of the destination host matches its own
• The destination host recognises a match and sends an ARP reply to the directly to the sending host with its MAC address.
• The ARP cache on both hosts is updated
• When the source host receives the reply, communication is established between them.

ARP Request for Remote Network

• The source host determines that the IP address it wants is not on the local network
• The local host checks its local route table for a path to the remote host or network.
• If no path is found, the source host determines the IP address of the default gateway and checks its ARP cache for an IP to MAC address mapping for the gateway.
• The source host sends the data packet to the router
• The router then handles the process beyond this point

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