ICMP echo
also: ping, ICMP echo request, ICMP echo reply, Echo Request, Echo Reply
A network diagnostic message that tests connectivity by sending a request packet and waiting for an echo reply from a remote host, commonly used by the ping command.
ICMP echo is part of the Internet Control Message Protocol (ICMP) and consists of two message types: Echo Request (type 8) and Echo Reply (type 0). When you send an echo request to a remote host, it responds by immediately sending back an identical echo reply, allowing you to verify that the host is reachable and measure round-trip latency.
The most common use of ICMP echo is the ping command. For example, ping 8.8.8.8 sends ICMP echo requests to Google's DNS server and displays the response times. Each reply confirms that the target host is online and can communicate with your system.
ICMP echo is a layer 3 (network layer) diagnostic tool that works independently of application-level services, making it useful for troubleshooting connectivity issues before assuming higher-layer problems. Some systems or firewalls disable ICMP echo for security reasons, which prevents ping from working but does not affect normal network traffic.