For anyone who's serious about investigating, diagnosing and exploring the way that computer networks work, there are a handful of tools that are always top of the list. Probably the best-known is Nmap. It's what you use to find out about all the devices on a network, so that you can then perhaps explore one or more of them in further detail.
As a way of understanding more about what's connected to your home network via your broadband router, it's a great tool. For checking that your computers aren't accessible to the world, it's invaluable.
For troubleshooting problems or finding the answers to complex questions, it's also handy. For example, you plug in a new device on your network such as a camera or a thermostat and you want to know what IP address it's been allocated and which of its ports are open for communication. A singe command in Nmap will scan your entire network in seconds and tell you. And if there are any devices on your network which you don't recognise, such as someone else using your wifi, it'll alert you to that situation too.
Nmap is officially a command-line tool, with a syntax that's notoriously difficult understand or remember. But thankfully the Windows version ships with an installer that also adds a GUI front end, so everything is on display and accessible with a few clicks.
You'll find the latest, brand new Nmap version 7 available to download now at https://nmap.org/download.html#windows and it runs to around 25 MB. The program is malware-free according to VirusTotal and Web of Trust. Note that the installer will also want to install something called WinPcap, which is harmless and necessary. You'll probably also receive a warning from your PC's firewall - again, this is safe to allow.
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