ARP

Address Resolution Protocol (ARP) is a telecommunications protocol used for resolution of network layer addresses into link layer addresses, a critical function in multiple-access networks. 

How it works

On an Ethernet/IP network when Host-A wants to send packet to Host-B it need to know the Host-B MAC address (MAC-B) in order to communicate. Host-A will ask for MAC-B with an ARP request packet sent in broadcast (FFFFFFFFFFFF). Only the machine with the specified IP address (Host-B) will answer to this request with an ARP reply packet sent back in uni-cast directly to the Host-A MAC address (MAC-A). At this point Host-A will send IP packets with destination IP-B using MAC-B as destination address in the Ethernet frame. ARP Request and Reply packet are sent only if the host doesn’t know the MAC address of the target machine; once learned the ARP Cache will be used.

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