Port Checker

The check runs from our server, so "open" here means "reachable from the internet." It is not the same as an open port on your computer: a service can run locally while a firewall, router, or hosting configuration blocks it from the outside.

Three states instead of two

• open: the connection succeeded, something is answering on the port;
• closed: the server refused the connection. The port is reachable, but no service is running on it;
• filtered: no answer until the timeout. This is how a silent firewall behaves.

The difference between closed and filtered changes what you do next: in the first case you look at the service, in the second at your filtering rules. Simple checks collapse both cases into "closed."

What answers on the port

The tool captures the service greeting: the OpenSSH version, the web server name, the mail server greeting, the MySQL version. From it you can see both the service itself and how current it is.

Well-known ports are labeled, and the ones that have no business being exposed are marked separately: databases, remote desktop, Redis.

Port sets

Instead of typing ports by hand, you can load a ready-made set: web server, mail, databases, remote access, games, or popular. The "my IP" link fills in your own address.

Limits

One run checks up to 100 ports, TCP only. Ranges like 1-65535 are not allowed: the tool checks specific services and does not work as a network scanner.