tcp/ip illustrated vol.1 (3) — ping and traceroute

The ping program sends an ICMP echo request to a host, expecting an ICMP echo reply to be returned.
Normally if you can’t Ping a host, you won’t be able to Telnet or FTP to that host. Conversely, if you can’t Telnet to a host, Ping is often the starting point to determine what the problem is.
Ping also measures the round-trip time to the host, give us some indication of how “far away” that host is.