The ICMP protocol can be used to test the availability of a particular destination. Figure shows ICMP being used to issue an echo request message to the destination device. If the destination device receives the ICMP echo request, it formulates an echo reply message to send back to the source of the echo request. If the sender receives the echo reply, this confirms that the destination device can be reached via the IP protocol.

The echo request message is typically initiated using the ping command as shown in Figure . In this example, the command is used with the IP address of the destination device. The command can also be utilized as shown in Figure using the IP address of the destination device. In these examples, the ping command issues four echo requests and receives four echo replies, confirming IP connectivity between the two devices.