Nmap is a free and open-source network discovery and security audit tool. In simple terms, network discovery is the process of finding out the devices that are on the network. Whether you are a network engineer or a penetration tester, Nmap is one of the most important tools in your arsenal.
It can help show what devices are connected to your network, can scan what ports they have got open, and also glean some additional information about these devices (eg. those -sY and -O flags). If you suspect a neighbour is using your home network, then this is a tool that will show that connected device.
It runs on Linux, MacOS and Windows and this guide really covers all the basics including the background to IP addresses and ports and how a device connects to a network.
See How To Use Nmap - A Comprehensive Guide: Basics To Advanced
Nmap is a free and open-source network discovery and security audit tool. Nmap stands for "Network Mapper".