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Ping uses the Internet Control Message Protocol (ICMP) to send messages across the network.


Most of the Internet tools (Telnet, FTP, Finger, Gopher, World Wide Web, and so on) communicate across the network using the Internet's Transmission Control Protocol (TCP). TCP is a complex protocol that breaks messages up into packets, sends them over the network, and puts them back together again at the other end. During transmission, packets may be lost, duplicated, or delayed, and TCP also includes mechanisms for detecting and dealing with these conditions.


Some applications, however, don't require all the complexity of TCP, so the Internet supports two other, simpler protocols: User Datagram Protocol (UDP) and Internet Control Message Protocol (ICMP).


Ping requests, for example, always fit into a single packet, and it is no big deal if that packet is lost, because the originating application can always try again a few seconds later if no response is received the first time.


Ping checks to see if another host is reachable by sending it a series of ICMP ECHO_REQUEST messages, and listening for the responses. All Internet hosts are required to respond to ICMP ECHO_REQUEST messages, so if you can't get through to a host with Ping, then it is pretty much guaranteed that none of the other Internet applications (such as Telnet) will work either.