Nmap (Network Mapper) | Free
Used to Scan Ports and Map Networks - and a whole bunch more!
Nmap is an abbreviation of 'Network Mapper', and it's very well known free open source hackers tool. Nmap is mainly used for network discovery and security auditing. Literally, thousands of system admins all around the world will use nmap for network inventory, check for open ports, manage service upgrade schedules, and monitor host or service uptime. Nmap, as a tool uses raw IP packets in creative ways to determine what hosts are available on the network, what services (application name and version) those hosts are providing information about, what operating systems (fingerprinting) and what type and version of packet filters/ firewalls are being used by the target. There are dozens of benefits of using nmap, one of which is that fact that the admin user is able to determine whether the network (and associated nodes) need patching. Nmap's been featured in literally every hacker movie out there, not least the recent Mr. Robot series. It's also worth mentioning that there's a GUI version of Nmap called 'Zenmap'. We'd advise you to learn using Nmap (i.e. the 'command line') then rotate into Zenmap when you are feeling all confident.