Which ports are tested
The scan covers 21 (FTP), 22 (SSH), 23 (Telnet), 25 (SMTP), 53 (DNS), 80 (HTTP), 110 (POP3), 143 (IMAP), 443 (HTTPS), 3306 (MySQL) and 8080 (HTTP alternate).
These are the ports where an unintended listener causes the most trouble. This is not an exhaustive scan -- there are 65,535 TCP ports and a service can run on any of them -- but an unexpected result among these eleven is worth investigating.
What an open port tells you
Open means something accepted a TCP connection. It does not mean the service is vulnerable, and it does not identify what is actually listening. Ports are a convention, not a guarantee: anything can run on 3306.
The results worth attention are services that should never face the public internet. Port 23 (Telnet) transmits credentials in clear text and has no business being open at all. Port 3306 exposed publicly means your database is reachable from anywhere, and should almost always be restricted to specific addresses or placed behind a VPN.
Why port 25 usually reads closed
Almost every cloud provider blocks outbound connections on port 25 to limit spam, and this tool runs on such a platform. Port 25 will therefore tend to read as closed regardless of whether the target actually accepts mail.
Use the SMTP Tester for questions about mail delivery -- it probes the submission ports as well and tells you explicitly when a result is inconclusive rather than genuinely negative.
Closed, filtered and the difference
This tool reports a port as open or not open, but there are really two ways to be not open. A closed port actively refuses the connection, which tells an attacker a host exists there. A filtered port silently drops the packet, so the connection attempt simply times out.
Filtering is the better posture for anything you do not intend to expose, because it gives away nothing. From the outside both look the same here -- the difference is how quickly the attempt fails.
Frequently asked questions
Is running this against a server I do not own legal?
A connection attempt to a public port is ordinary client traffic, but acceptable-use policies and local law vary, and repeated scanning of third-party hosts is widely treated as hostile. Only test hosts you own or have written permission to test.
Why does a port show closed when the service is running?
Most often a firewall between this server and yours is dropping the connection, or the service is bound to localhost rather than a public interface. Cloud security groups are the usual culprit.
Should I close every port that shows as open?
No. Close the ones nothing needs. A web server must have 443 open, and closing it takes the site offline. The goal is that every open port corresponds to a service you intend to expose.
Does this detect what software is running?
No. It reports only whether a TCP connection was accepted. It does not read service banners or attempt version detection.