Open Port Checker - Test If a Port Is Open Online

What Is an Open Port Checker?

Servers listen for connections on numbered ports. A web server listens on 443, a mail server on 25 or 587, a database on 3306 or 5432. When a port is open, something is listening behind it and accepting connections. When it is closed or filtered, connection attempts fail — either rejected outright or dropped silently by a firewall.

An open port checker attempts a connection to a specific port from outside your network and reports whether it succeeded. This external vantage point is what makes it useful. A service can be running perfectly on the machine while a firewall, a security group, or an ISP block prevents anyone else from reaching it. Testing from the server itself will not reveal that; testing from outside will.

How to Use This Tool?

1. Enter an IP address or hostname
2. Enter the port number you want to test
3. Run the check

If you have a domain rather than an address, the tool will resolve it first. To see which address it resolves to before testing, run a DNS lookup on the domain.

Understanding the Results

Open means a connection was established. Something is listening on that port and accepting traffic from outside. For a service you intended to expose, this is the confirmation you wanted.

Closed means the host is reachable but nothing is listening on that port. The connection was actively refused, which is a definite answer — the machine is up, the service is not running or not bound to that port.

Filtered or timed out means no answer came back at all. A firewall dropped the packet without responding. This is the harder case to diagnose, because it looks identical whether the block sits at the server firewall, a cloud security group, or the network in between.

Common ports worth knowing: 22 for SSH, 25, 465 and 587 for mail submission, 53 for DNS, 80 and 443 for web traffic, 3306 for MySQL, 5432 for PostgreSQL, 3389 for remote desktop.

Ports that should not be open to the world are worth checking on your own servers. A database port reachable from any address is a serious exposure — 3306 and 5432 should normally be restricted to specific addresses or accessible only over a private network or VPN. The same applies to 3389, which is a persistent target for automated attacks.

When Is This Tool Useful?

• Confirming a newly configured service is reachable from outside the server
• Checking whether a firewall rule took effect as intended
• Verifying that a database or admin port is not exposed publicly
• Diagnosing why a mail server will not send, by testing whether the submission port is reachable
• Confirming that a hosting provider or ISP is not blocking a port you need
• Checking that a web server is listening on 443 before troubleshooting the certificate — if the port is open but HTTPS still fails, validate the certificate next
• Testing a self-hosted service after a router port-forward change

What This Tool Does Not Do

This is a single-port check from one location, not a security audit. It tells you whether one port accepted a connection at one moment. It does not enumerate every open port, identify the software behind them, or assess whether that software is patched.

The result also depends on where the request comes from. A server that permits connections only from specific addresses will show as filtered here while working perfectly for the addresses on its allowlist. A filtered result on your own server may simply mean the allowlist is doing its job.

Only test hosts you own or have permission to test. Scanning other people's infrastructure is unwelcome at best and unlawful in many jurisdictions.

Frequently Asked Questions

How do I check if a port is open on my server?

 Enter the hostname or IP above, add the port number, and run the check. The test connects from an external server, which is the part that matters — a port can be reachable from the machine itself or from inside your network while still being closed to the internet.

Which port numbers should I be checking?

The common ones: 80 and 443 for web traffic, 22 for SSH, 25, 587, and 465 for mail, 3306 for MySQL, 5432 for PostgreSQL, 3389 for remote desktop, and 21 for FTP. If a service is not responding, check the port it is configured to use rather than the default — they often differ.

How do I check if port 25 is blocked?

Run the check against your mail server on port 25. Most residential ISPs and many cloud providers block outbound 25 by default to limit spam, which is why mail sends fine from one network and silently fails from another. If 25 is blocked, 587 with authentication is the normal alternative.

How do I open a port that shows as closed?

Work through three layers in order: confirm the service is actually running and bound to 0.0.0.0 rather than 127.0.0.1, then check the host firewall, then check the cloud provider's security group or network ACL. A port can be blocked at any of the three, and the symptom looks identical from outside.

What is the difference between closed and filtered?

 Closed means the host actively refused the connection — it is reachable, but nothing is listening on that port. Filtered means no response came back at all, because a firewall dropped the packet silently. Closed is a clear answer; filtered means something is blocking it.

Why does a port show as closed when my service is running?

The service may be bound to localhost only rather than to all interfaces, so it accepts connections from the machine itself but not from outside. A firewall rule or cloud security group blocking the port produces the same symptom.

Which ports should never be open to the internet?

Database ports such as 3306 and 5432, and remote desktop on 3389, should normally be restricted to specific addresses or reachable only over a private network. Leaving them open to everyone invites automated attacks.

Is it legal to scan any host?

Test only hosts you own or have explicit permission to test. Unauthorised port scanning is against the terms of most providers and unlawful in many jurisdictions.