When a system is hiding from ping sweeps, it can be difficult to know what's online. Fortunately, Nmap provides a ping agnostic method for scanning that can be very beneficial to figure out some of these issues.

When Nmap runs a "normal" scan, it will first run a ping sweep and then follow up with actual port scans (of whatever port ranges specified). If hosts are not responding to a ping, they won't be fully scanned—which means that even if they have services online, those services will not be detected. When running a port scan, missing services or hosts is a very serious problem!

By running a scan with the -Pn flag, Nmap will completely skip running the initial ping sweep, and will scan all hosts in the specified target range. Although this generally takes longer to run—since scanning hosts that are really offline is a big waste of time—it is extremely useful to find hosts that may otherwise have been missed.

You can easily see in the preceding screenshot that...