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.