The Address Resolution Protocol is fairly straightforward. Each device on a network contains two types of addresses: an IP address and a MAC address. This protocol was developed to help resolve an IP address to a unique MAC address.

Each device on the network has an IP address that is capable of changing depending on DHCP settings or IP configuration. Each device also has a unique Media Access Control address that is ‘burned’ into each network interface card. ARP matches IP addresses to MAC addresses and vice versa using four message types: ARP Request, ARP Reply, Reverse ARP (RARP) Request, and RARP Reply.

An ARP Request is used when a device wants to know who is using a specific IP address.

An ARP Reply is just what it says. It is a reply to the ARP Request that informs the requesting device that is has the requested IP address and also includes its MAC address. Most of the time when devices do not possess the requested IP address that is within the ARP Request they will ignore the request and only the device containing the requested IP address will issue an ARP Reply.

A RARP Request is just the opposite of an ARP Request. The requesting device wants to know who is using a specific MAC address.

A RARP Reply informs the device issuing a RARP Request that it is using this MAC address and includes its IP address as well. Again, a RARP Reply works in much the same manner as an ARP Reply.

Each device contains an ARP table. This table has a short life span and contains a list of matched IP and MAC addresses. This allows networking devices to avoid broadcasting ARP and RARP Requests across the network which reduces network overhead and increases network throughput.

References: RFC 826 and RFC 903