Test whether a specific TCP port is open on any public host. Our free port checker is perfect for firewall debugging, verifying services like SSH, HTTPS, SMTP, and MySQL are reachable from the public internet, and diagnosing 'connection refused' errors.
Our port checker analyzes global network conditions, with a strong emphasis on data from probes located across the United States, providing precise regional insights.
The test is deliberately simple, which is what makes it trustworthy. From our server we open a TCP connection to the host and port you enter and watch what the remote end does. A completed three-way handshake means a service is listening and accepting connections from the public internet — reported as open. An immediate RST packet means the host is reachable but nothing is bound to that port, or a firewall is actively refusing it — reported as closed.
Open means a listening service completed the handshake — if that port is a database, cache or admin interface, treat it as an urgent finding rather than a success. Closed means the host answered but rejected the connection, which usually means the service is stopped or bound only to 127.0.0.1 instead of 0.0.0.0.
| Port | Service | Notes |
|---|---|---|
| 22 | SSH | Remote shell |
| 25 / 465 / 587 | SMTP / SMTPS / Submission | Email — 25 often blocked |
| 80 | HTTP | Unencrypted web |
| 443 | HTTPS | TLS web |
| 3306 | MySQL | Should not be public |
| 5432 | PostgreSQL | Should not be public |
| 6379 | Redis | Should not be public |
| 27017 | MongoDB | Should not be public |
| 8080 / 8443 | Alt HTTP / HTTPS | App servers, admin UIs |
A webmaster uses the port checker to confirm that port 80 (HTTP) or 443 (HTTPS) is open on their server's public IP address. This ensures that their website is accessible to visitors globally and that no firewall rules are inadvertently blocking web traffic, promptly identifying any connectivity issues.
A developer deploys a new application that listens on a specific, non-standard port (e.g., a gaming server on port 7777). They use the online port scanner to verify that this port is open externally, confirming that users can connect to their application through the internet, often after configuring port forwarding.
An IT administrator modifies firewall rules to allow a new service. They perform a firewall test with the port checker to ensure that the intended ports are indeed open and all other sensitive ports remain closed, validating the security policy and preventing unintended exposures. This is a critical `is port open` check.
A gamer wants to host a multiplayer game but friends can't join. They use an open port test to check if the game's required port (e.g., 27015 for Counter-Strike) is open on their home network's public IP. If closed, they know to adjust router settings for port forwarding.
Yes — free, unlimited tests, no signup.
A port checker sends a TCP SYN packet to the target port. If the target responds with a SYN-ACK packet, it means a service is listening, and the port is open. If no response or an RST packet is received, the tool concludes the port is closed or blocked by a firewall.
Yes, but you must test your public IP address, not your local network IP (e.g., 192.168.1.100). For an external port check to succeed on your computer, you would need to configure port forwarding on your router to direct traffic from your public IP to your computer's internal IP.
Most basic online port checkers primarily focus on TCP ports because TCP connections involve a handshake, making it easier to determine port status. Checking UDP ports is more complex as it's a connectionless protocol, and tools often rely on application-layer responses for definitive status.
If your ISP blocks certain ports, an external port checker will report those ports as closed or filtered, even if you've configured your local firewall and service correctly. In such cases, you would need to contact your ISP or route traffic through a VPN that uses an unblocked port.
Regularly performing a firewall test helps to ensure that your security configurations are working as intended. It allows you to quickly identify any unintended open ports that could expose your network to threats and confirms that necessary services remain accessible, maintaining both security and functionality.
Use this free online port checker to verify firewall rules, confirm a newly deployed service is reachable and debug SSH, mail or database connectivity from outside your network. Follow up with the HTTP header checker for web-layer issues and the down checker for full-site availability.