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ICMP echo messages (ICMP type 8) are sent to a remote computer and are returned in an echo-reply response. The primary use for these messages is to check the availability of the target computer.

1. Computer A creates an ICMP ECHO datagram, using computer A's IP address as the source IP address, and computer B's IP address as the destination.
2. The ICMP ECHO datagram is transmitted via the network to the destination computer 'B'.
3. The destination computer (computer B) copies the ECHO information into a new ECHO-REPLY message datagram.
4. The destination computer (computer B) destroys the original ICMP ECHO message.
5. The destination computer (computer B) now becomes the source of a new ECHO-REPLY datagram. Host B places it's own address in the source IP address field of the IP header, and host A's IP address in the destination field of the IP header.
6. The datagram is transmitted to the network and is routed to computer A.

Visual Example of ICMP Echo:

That's how ping works, by the way... See the full ping tutorial in our troubleshooting tutorials for more information.