The basic function of the ARP protocol is to resolve IP addresses to Media Access Control (MAC) addresses. When a system attempts to contact another host, IP first determines whether the other host is on the same network it is on by looking at the IP address. If IP determines that the destination is on the local network, it consults the ARP cache to determine whether it has a corresponding entry.
If there is not an entry for the host in the ARP cache, IP sends a broadcast on the local network, asking the host with the target IP address to send back its MAC address. The communication is sent as a broadcast because without the target system's MAC address, the source system is unable to communicate directly with the target system.
The Reverse Address Resolution Protocol (RARP) performs the same function as ARP, but in reverse. In other words, it resolves MAC addresses to IP addresses. RARP makes it possible for applications or systems to learn their own IP address from a router or DNS server. Such a resolution comes in handy for tasks such as performing reverse lookups in DNS.
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