Test if a TCP port is open

Port Check Tool: Test if a TCP Port Is Open

A port check tells you whether a server is reachable on a specific TCP port. That simple “open” or “closed” result can save hours of troubleshooting when a service appears offline. If a website loads slowly, an API times out, or a mail server cannot accept incoming connections, the first step is to confirm that the target host is listening on the correct port and that a firewall is not blocking traffic.

This online port check tool lets you test a hostname or IP address from the public internet. Enter the host, specify a port number, and run the check. The result indicates whether a TCP connection could be established. It does not log in to the service or inspect application-level responses; it simply verifies reachability at the network layer.

What a port check can (and cannot) tell you

An “open” result means the port is reachable and a service accepted the connection. It does not guarantee that the application is healthy or that authentication will succeed. A “closed” result usually indicates that no service is listening or that a firewall rejected the connection. Timeouts can indicate packet filtering, routing problems, or intermittent network instability.

Because this tool checks connectivity from an external network, it is useful for verifying public exposure. If a port appears open locally but closed externally, the issue is likely at the perimeter firewall, cloud security group, or NAT gateway. This is common with newly deployed servers, recent firewall rule changes, or misconfigured load balancers.

Common ports to test

• 80 / 443 for HTTP and HTTPS web services.
• 22 for SSH administration (keep restricted).
• 25 / 465 / 587 for SMTP and mail submission.
• 53 for DNS services when hosting your own name servers.
• 3306 / 5432 for database servers (often should not be public).

Troubleshooting steps when a port is closed

First, verify that the application is running and bound to the correct interface. Services that listen only on 127.0.0.1 will not be reachable from the public internet. Next, check host firewalls (like ufw, firewalld, or Windows Firewall) and cloud security rules. Finally, confirm that any load balancers or proxies are configured to forward traffic to the right backend port.

If the port is open but clients still fail, move up the stack: check TLS certificates, application logs, and authentication settings. Pair this port check with DNS lookup and ping tests to validate the full path from name resolution to network reachability.

Use cases for teams and operators

Port checks are useful before go-live events, after firewall changes, and when monitoring vendor services. They help security teams confirm that only intended ports are exposed and assist DevOps teams when validating new deployments. For SaaS integrations, a quick port test can confirm whether a third-party endpoint is reachable before opening a support ticket.

If you are managing a multi-region system, you can check the same port on multiple IPs to verify that every region is configured consistently. This prevents a common problem where only one zone has the correct firewall rule and traffic to other zones fails silently.

Security and responsible testing

Only test ports on systems you own or have permission to evaluate. Port scanning without authorization can violate acceptable use policies or local laws. This tool performs a single target check at a time to help you validate connectivity responsibly. If you need large-scale testing, use approved security tools in a controlled environment.

Frequently asked questions

Why is a port open but the website still fails? The port check verifies a TCP connection only. Application errors, TLS mismatches, or DNS issues can still prevent a site from loading correctly.

Does a closed port mean the server is down? Not always. The server may be online but not listening on that port, or a firewall may be blocking traffic.

Can I test UDP ports? This tool focuses on TCP reachability. UDP testing requires different methods and may produce ambiguous results.

Use this port check tool as part of a broader diagnostic workflow. When combined with DNS lookup, propagation testing, and ping, it gives you a clear view from name resolution to service availability.