Port Checker: Test if a TCP Port Is Open
What a port check tells you
A port checker tests whether a host is reachable on a specific TCP port from the public internet. It only confirms that something accepted a connection on that port; it does not validate the application protocol (HTTP, SSH, etc.).
It is useful for debugging firewall rules, security groups, load balancers, and "can't connect" issues. If the port is closed, the problem is often network or configuration; if it is open, the next step is to test the service itself.
For HTTP/HTTPS services, combine this with an HTTP headers check to see status and headers. For mail, use SMTP test and MX lookup. For hostname resolution, use DNS lookup.
How to use it
Enter a hostname or IP and a port number (e.g. 443 for HTTPS, 22 for SSH, 587 for SMTP). Run the check.
If the port is closed or the connection times out, check firewall rules, security groups, and whether the service is actually listening on that port. Ensure the host resolves correctly (use DNS lookup if needed).
The tool reports latency and a suggested service name for common ports (e.g. 443 → https). Use that as a hint, not a guarantee—many services use non-standard ports.
Interpreting results
An open port means the host accepted a TCP connection. It does not guarantee that the service is healthy or that it speaks the protocol you expect. Run an application-level check (e.g. HTTP request, SMTP EHLO) to confirm.
Closed or timeout can mean: the service is down, a firewall is blocking, the port is wrong, or the host is unreachable. Verify DNS and try from another network if possible.
After confirming reachability, use HTTP or API checks to validate app behavior, and consider monitoring so you are alerted when the port becomes unreachable.
Common use cases
Verifying that a new server or container is reachable from the internet after a deploy, debugging VPN or firewall rules that might block certain ports, and checking that a migration did not leave the old port closed.
For email, SMTP test goes beyond a simple port check by performing an SMTP handshake and reporting STARTTLS support.