| The Nmap Tool The nmap tool is a freeware tool that is very popular among hackers, but it can be very useful for its legitimate purpose: to assist in the administration of your networks. Hackers primarily use it as a port scanning tool against networked devices to gather information about every port or specific ports. It can also scan a system and attempt to determine what OS it is to report back to the user. Knowing which operating system a network is using can help the hacker when determining how to break into a network. The main ports that it checks for a Cisco router are 1-25, 80, 512-515, 1993, 1999, 2001, 4001, 6001, and 9001.