Nmap (Network mapper) is an open-source tool for network and security auditing. It is used for host and service discovery on networks and has a wide range of use cases. It can scan ports, discover live hosts, detect service and OS versions, runs vulnerability scans, and can be used with many scripts.
I'll show you the basics of nmap in this post. This is more than enough to get started.
Source: Getting started with nmap.