Two other protocols in the IP suite perform important functions, although these are not directly related to the transport of data: Address Resolution Protocol (ARP) and Internet Control Message Protocol (ICMP). ARP and ICMP are maintenance protocols that support the IP framework and are usually invisible to users and applications.
IP packets contain both source and destination IP addresses, but the hardware address of the destination computer system must also be known. IP acquires a system's hardware address by broadcasting a special inquiry packet (an ARP request packet) containing the IP address of the system with which it is attempting to communicate. All of the ARP-enabled nodes on the local IP network detect these broadcasts, and the system that owns the IP address in question replies by sending its hardware address to the requesting computer system in an ARP reply packet. The hardware/IP address mapping is then stored in the requesting system's ARP cache for subsequent use. Because the ARP reply can also be broadcast to the network, it is likely that other nodes on the network can use this information to update their own ARP caches. (You can use the arp utility to view the ARP tables.)
ICMP allows two nodes on an IP network to share IP status and error information. This information can be used by higher-level protocols to recover from transmission problems or by network administrators to detect network trouble. Although ICMP packets are encapsulated within IP packets, they are not considered to be a higher-level protocol (ICMP is required in every TCP/IP implementation). The ping utility makes use of the ICMP echo request and echo reply packets to determine whether a particular IP node (computer system) on a network is functional. For this reason, the ping utility is useful for diagnosing IP network or gateway failures.