Nmap is a tool that can perform network scanning and enumeration by sending packets to hosts and analyzing their responses. The command Nmap -p 445 -n -T4 --open 172.21.0.0/16 would scan for SMB port 445 over a /16 network with the following options:
-p 445 specifies the port number to scan.
-n disables DNS resolution, which can speed up the scan by avoiding unnecessary queries.
-T4 sets the timing template to aggressive, which increases the speed of the scan by sending packets faster and waiting less for responses.
-open only shows hosts that have open ports, which can reduce the output and focus on relevant results. The other commands are not optimal for scanning SMB port 445 over a /16 network when stealth is not a concern and the task is time sensitive.