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Tools

Open Port Checker

Network and domains

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Is the port open: state, response, service.

We test the TCP connection from our server. “Filtered” means the port did not respond at all — usually a firewall is blocking it.

Why check whether a port is open

A port is the door through which a service accepts incoming connections. When it is shut the service still runs, but nobody can reach it.

The site will not open The web server is running but port 80 or 443 is blocked by a firewall. The usual reason behind “the server is up but the site is gone”.
Mail will not send Providers often block port 25 on home connections to stop spam. Use 587 or 465 instead.
The server is unreachable from outside Connections fail: the port is not forwarded in the router, or the provider blocks it upstream.
Verifying a change You edited a firewall rule and want to confirm it works from the outside, not just inside your own network.

The ports you meet most often

80, 443Web: plain http and secure https. With 443 closed the site will not load over https.
25, 465, 587Sending mail. Port 25 is often blocked by providers, so 587 with encryption is the practical choice.
110, 143, 993, 995Receiving mail: POP3 and IMAP, plain and secure variants.
21, 22File transfer: FTP and SSH. The latter is safer, so 21 is often disabled on purpose.
3306, 5432MySQL and PostgreSQL databases. These should be closed to the outside — an exposed database port is a serious risk.

A closed port is not always a problem

Before opening one, work out whether it really needs to be open.

Open

When it makes sense

  • A web server must accept visitors
  • A mail server needs to send
  • A service people connect to remotely
vs
Leave shut

When to leave it alone

  • A database port belongs to the server only
  • Control panel admin interfaces
  • Anything you did not configure yourself

Every unnecessary open port is attack surface. Bots scan addresses around the clock and probe exactly the standard ports. Open only what genuinely has to accept outside connections.

FAQ

What is the difference between “closed” and “filtered”?
They are different things, and the distinction matters when configuring a server. “Closed” means the server actively refused: the port is reachable but nothing is listening on it. “Filtered” means there was no reply at all — the packet vanished. That is how a firewall behaves when it silently drops connections. If you have just set up a service and see “filtered”, look at your firewall rules rather than the application.
Why is my port open locally but closed in this check?
There are three usual causes. First: the service listens only on 127.0.0.1 instead of 0.0.0.0 — check the configuration; in MySQL this is the bind-address setting. Second: the port is blocked by the server firewall or a cloud security group. Third: your provider blocks the port upstream — this is common with port 25 to limit spam.
Which ports are dangerous to leave open?
Database ports first of all: 3306 (MySQL), 5432 (PostgreSQL), 6379 (Redis), 27017 (MongoDB). They should not face the internet — database access belongs on the server itself or through an SSH tunnel. Redis and MongoDB historically shipped with no password at all, so an open port means anyone can read the data. It is also worth closing 3389 (RDP) and 5900 (VNC), or at least restricting them by IP.
Is port scanning legal?
Checking your own servers is normal administrative work. Mass scanning of infrastructure you do not own is treated in many countries as reconnaissance ahead of an attack and can have consequences. Our tool is deliberately limited to 15 ports per request and uses no aggressive techniques — it is for diagnosing your own server, not for scanning networks.
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