A knock.
An echo.

Send your first signal.

Open a terminal and replace HOST with a hostname or IP address you are authorized to test. No URL scheme or port is needed.

ping -c 4 HOST

ping -n 4 HOST

Read the reply, not the tea leaves.

A reply confirms that an ICMP round trip succeeded at that moment. The reported time is the journey there and back, not a one-way measurement. Packet loss describes how many of your echo requests received no reply.

Silence is not a verdict.

Firewalls may block ICMP, and hosts may limit their replies. A machine that ignores ping can still serve a perfectly good website. Try an HTTPS request or a TCP connection before declaring it missing.