On an IPV4 network, the most common types of ARP messages are request/replies:

• A standard ARP request is a broadcast message that is sent out on the network requesting a resolution of an IP address to a MAC address.
• A standard ARP reply is a unicast message that is sent to the requesting host that provides the address resolution.

However, as we'll see next, there are a few other types of ARP messages. We'll start with the Reverse Address Resolution Protocol (RARP), which is the reverse of ARP.