Not directly related to data transport, but important nonetheless, the Address Resolution Protocol (ARP) is one of the maintenance protocols that supports the TCP/IP suite and is usually invisible to users and applications.
If two systems are to communicate across a TCP/IP network, the system sending the packet must map the IP address of the final destination to the physical address of the final destination. IP acquires this physical address by broadcasting a special inquiry packet (an ARP request packet) containing the IP address of the destination system. All ARP-enabled systems on the local IP network detect these broadcast messages, and the system that owns the IP address in question replies by sending its physical address to the requester (in an ARP reply packet). The physical/IP address is then stored in the ARP cache of the requesting system for subsequent use.
Because the ARP reply can also be broadcast to the network, other systems on the network can use this information to update their own ARP caches. (Use the arp program to view the ARP tables.)