Nmap
FreeSee what is actually on your network and which ports are open
A network discovery and port scanning tool. Scan a range to find live hosts, probe open ports, identify services and versions, and infer operating systems. It is commonly used for inventorying home and corporate networks, verifying firewall rules and diagnosing connectivity. A graphical front end, Zenmap, is included.
Why we picked it
Most people know remarkably little about their own home network: how many devices are attached to the router? What ports does that smart speaker have open? Is a service on the NAS accidentally exposed to the internet? Does that cheap camera have an administration port anyone can reach?
Nmap has been the standard tool for answering these questions for nearly thirty years.
Common usage:
List live devices on your home subnet:
nmap -sn 192.168.1.0/24
See which ports a device has open and what is running:
nmap -sV 192.168.1.100
Check what your own server exposes, scanning your public IP from outside:
nmap -Pn your-server-ip
Its real value is verifying that reality matches your assumptions. You believe the firewall closed a port — a scan tells you whether it actually did. You believe the NAS is LAN-only — a scan from outside tells you whether it leaked. A great many security incidents begin with "I thought that was closed."
It also performs service version detection (which nginx or SSH build is running), operating system fingerprinting, and finer checks through the NSE scripting engine.
The usage boundaries matter more than the features:
- Only scan networks and hosts you own or have written authorization to test. Your home network, servers you rent, and corporate assets you are responsible for are normal operational work.
- Scanning other people's networks without authorization is illegal in many jurisdictions, even if you "only looked". In corporate environments it also commonly violates internal policy.
- Some cloud providers and ISPs treat scanning as a precursor to attack and will suspend accounts or addresses. Read the terms of service before scanning from a cloud instance.
Note the licence as well: Nmap uses the Nmap Public Source License, not standard GPL. Personal and internal use is entirely free, but embedding it in a commercial product you sell requires a separate OEM licence. Read the licence before redistributing it in any project.
A practical tip: if the command line is unfamiliar, the bundled Zenmap GUI offers common scan profiles — start there and read the commands it generates to learn the flags.