In order to scan a host effectively, it's important to first understand how to detect hosts that are "alive" or online. Because many system administrators try to hide their systems from the Internet, certain hosts will appear to be offline until further probed. Fortunately for us, Nmap has several ways to detect which hosts are online.

The most straightforward way to detect hosts is to run a ping sweep. A ping—or an ICMP echo request that machines are designed to respond to—is a simple "are you there?" question and answer conversation.

Pings were named after sonar—the underwater "pings" that submarines send to detect other ships and submersibles in the area—and work in a similar way for computers. While you can test the ping command very easily by simply typing ping google.com, using Nmap for ping sweeps can allow significant efficiency gains across larger target network scopes.

It's easy to run a ping only sweep with Nmap using the -sn flag. This makes sure to run only...