ICMP (Internet Control Message Protocol)

ICMP (Internet Control Message Protocol) is used by network devices to send error and diagnostic messages, such as "destination unreachable" or "time exceeded". It works alongside IP and does not carry application data.

The ping command uses ICMP Echo Request and Echo Reply messages to check whether a host is reachable and measure the round-trip time. Traceroute (tracert on Windows) uses the "time exceeded" messages to list every router along a path. Some firewalls block ICMP, which is why a reachable server may not answer pings.