Hosts protected by packet filtering systems, such as firewalls or intrusion prevention systems
sometimes cause incorrect results because of rules that are used to block certain types of
traffic. The flexibility provided by Nmap in these cases is invaluable, since we can easily try
an alternate host discovery technique (or a combination of them) to overcome these
limitations. Nmap also includes a few very interesting features to make our traffic less
suspicious. For this reason, learning how to combine these features is essential if you
want to perform really comprehensive scans