What "open" actually confirms — and what it doesn't

A successful TCP connection to a port confirms exactly one thing: something is listening and willing to complete a TCP handshake on that port from this server's network position. It says nothing about whether the service behind that port is healthy, correctly configured, or actually the service you expect to be there — a port can be "open" while the application behind it is broken, misconfigured, or even a completely different service than intended. This tool is a reachability check, the first and cheapest diagnostic step, not a substitute for actually testing the service's real behavior (an HTTP request, a TLS handshake, an application-level health check) once reachability is confirmed.

Why closed and filtered look identical from outside

From this tool's vantage point, a port with nothing listening on it (actively refused, usually fast) and a port silently dropped by a firewall (times out, usually slow) can be hard to distinguish without careful timing analysis — and even then, some firewalls deliberately mimic a closed port's fast-reject behavior specifically to avoid revealing that filtering is happening at all. That's exactly why the guide above cautions against assuming "closed" means "nothing is running there" — it might mean a service is running but a firewall is blocking this specific connection path, which is a very different situation requiring a different fix (adjusting firewall rules) than an actually-down service would.

Why the common-ports scan checks a fixed list, not an arbitrary range

Scanning every one of the 65,535 possible ports on a host would take a long time and starts to resemble port-scanning behavior that many hosting providers and intrusion detection systems flag or rate-limit — which is exactly the kind of activity this site's abuse-prevention measures are designed to avoid enabling. Checking a fixed list of ~27 well-known service ports (HTTP, HTTPS, SSH, common database ports, and so on) covers the overwhelming majority of what anyone actually wants to check quickly, while staying well within the bounds of a reasonable diagnostic tool rather than a general-purpose scanning utility.