The most common ICMP message types are given in the following table:

The address resolution protocol (arp) is a protocol used by the Internet Protocol (IP) [RFC826] and specifically IPv4, to map IP network addresses to the hardware addresses used by a data link protocol. The protocol operates below the network layer as a part of the interface between the OSI network and OSI link layer and it is used when IPv4 is used over Ethernet.

The term address resolution refers to the process of finding an address of a computer in a network and the address is "resolved" using a protocol in which a piece of information is sent by a client process executing on the local computer to a server process executing on a remote computer and the information received by the server allows the server to uniquely identify the network system for which the address was required and therefore to provide the required address. The address resolution procedure is completed when the client receives a response from server containing the required address.

There are four types of arp messages that may be sent by the arp protocol and these are identified by four values in the "operation" field of an arp message. The types of message are:

1. ARP request
2. ARP reply
3. RARP request
4. RARP reply

The format of an arp message is shown below:

To reduce the number of address resolution requests, a client normally caches resolved addresses for a (short) period of time and the arp cache is of a finite size, and would become full of incomplete and obsolete entries for computers that are not in use if it was allowed to grow without check. The arp cache is therefore periodically flushed of all entries. This deletes and unused entries frees space in the cache and it also removes any unsuccessful attempts to contact computers which are not currently running.

The figure below shows the use of arp when a computer tries to contact a remote computer on the same LAN (known as "sysa") using the "ping" program and it is assumed that no previous IP datagrams have been received form this computer, and therefore arp must first be used to identify the MAC address of the remote computer.