A ping scan involves sending an ICMP ECHO Request to a host. If a host is live, it will return an ICMP ECHO Reply, as shown in the following diagram:
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ICMP request and reply
The operating system's ping command provides the facility to check whether the host is live or not. Consider a situation where you have to test a full list of IP addresses. In this situation, if you test the IP addresses one by one, it will take a lot of time and effort. In order to handle this situation, we use ping sweep.