Internet Control Message Protocol (ICMP)

ICMP is a protocol that works with IP to provide error checking and reporting functionality. In effect, ICMP is a tool that IP uses in its quest to provide best-effort delivery. ICMP functions at the network layer of the OSI model.

ICMP can be used for a number of functions. Its most common is probably the widely used and incredibly useful ping utility. ping sends a stream of ICMP echo requests to a remote host. If the host is able to respond, it does so by sending echo reply messages back to the sending host. In that one simple process, ICMP enables the verification of the protocol suite configuration of both the sending and receiving nodes and any intermediate networking devices.

Address Resolution Protocol/Reverse Address Resolution Protocol (ARP/RARP)

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.