ARP protocol consists of the following 4 basic messages:
1. ARP request : Computer ‘A’ asks on the network, “who has this IP?”
2. ARP reply : All the other computers ignore the request except the computer which has the requested IP. This computer, lets say ‘B’ says, I have the requested IP address and here is my MAC address.
3. RARP request: This is more or less same as ARP request, the difference being that in this message a MAC address is broad-casted on network.
4. RARP reply : Same concept. Computer ‘B’ tells that the requested MAC is mine and here is my IP address.
All the devices that are connected to network have an ARP cache. This cache contains the mapping of all the MAC and IP address for the network devices this host has already communicated with.