Nmap's port registration file(nmap-services)contains empirical data about howfrequently each TCP or UDP port is found to be open. This data wascollected by scanning tens of millions of Internet addresses, thencombining those results with internal scan data contributed by largeenterprises. By default, Nmap scans the 1,000 most popular ports ofeach protocol it is asked to scan. Alternatively, you can specifythe -F (fast)option to scan only the 100 most common ports in each protocolor --top-ports to specify an arbitrary number of ports toscan.