Discovery Options Overview
Before port scanning a target, Nmap will attempt to send ICMP echo requests to see
if the host is “alive.” This can save time when scanning multiple hosts as Nmap will
not waste time attempting to probe hosts that are not online. Because ICMP
requests are often blocked by firewalls, Nmap will also attempt to connect to port
80 and 443 since these common web server ports are often open (even if ICMP is
not).
The default discovery options aren’t useful when scanning secured systems and can
hinder scanning progress. The following section describes alternative methods for
host discovery which allows you to perform more comprehensive discovery when
looking for available targets.
