Tenable Nessus

Nessus is the vulnerability scanner from Tenable. You point it at a range of addresses, and it probes each device for open ports, running services, missing patches, default passwords and known flaws, using a large and frequently updated library of checks called plugins. Scans can run with credentials, which lets Nessus log in and see installed software and settings far more accurately than an outside scan.

Nessus Essentials is a free edition with a limit on how many addresses it can scan, aimed at learning and small networks. Nessus Professional is the paid edition used by consultants and IT staff. Companies that want continuous scanning, dashboards and asset tracking usually move to Tenable's cloud product, Tenable Vulnerability Management.

Only scan networks you own or have written permission to test. Run a first scan out of hours, since some checks can upset old printers or fragile devices, and decide who will fix what the report finds.