If a host is blocking a ping request, Nmap may detect that the host is not active; so, Nmap may not perform heavy probing, such as port scanning, version detection, and operating system detection. To overcome this, Nmap has a feature for disabling host discovery. With this option, Nmap will assume that the target machine is available and will perform heavy probing against that machine.

This option is activated using the -Pn option.