Perhaps the most basic network monitoring tool is Ping. Ping uses the Internet Control Message Protocol (ICMP) to check whether a host is reachable. It also commonly reports the total round-trip time (latency). Ping is available as a built-in command or utility for all common operating systems.
The ICMP defines a set of special IP messages that are typically useful for diagnostic and control purposes. Ping works by using two of these messages: echo request and echo reply. The Ping utility sends an echo request ICMP message to a destination host. When that host receives the echo request, it should respond with an echo reply message.
When the echo reply is received, Ping knows that the destination host is reachable. Ping can also report the round-trip time from when the echo request was sent to when the echo reply was received. ICMP echo messages are usually small and easy to process, so this round-trip time often serves as a best-case estimate of network latency.