NetCheck Tools

Port Checker

Test whether a TCP port on a public host is open, closed or filtered. One host, one port, one real connection attempt — with latency measurement.

Understanding the three results

  • Open: a service accepted the TCP connection. The port is reachable from the internet.
  • Closed: the host answered with a refusal (TCP RST). The machine is reachable but nothing listens on that port.
  • Timeout / filtered: no answer at all within 3 seconds. A firewall is dropping packets, or the host is down.

Common ports to test

22 SSH, 25 SMTP, 53 DNS, 80 HTTP, 110 POP3, 143 IMAP, 443 HTTPS, 465/587 SMTP submission, 993 IMAPS, 3306 MySQL, 5432 PostgreSQL, 6379 Redis, 8080 alternative HTTP. As a rule, databases and admin interfaces should not be open to the whole internet — if this tool shows them open, restrict them.

Testing port 443? Follow up with the SSL checker to verify the certificate being served.

Frequently asked questions

›What does the port checker do?

It attempts a real TCP connection from our server to the host and port you enter. “Open” means the connection was accepted, “closed” means the host actively refused it, and “timeout/filtered” means nothing answered — usually a firewall silently dropping packets.

›Can I scan multiple ports or a port range?

No, by design. This tool tests exactly one host and one port per request. It is a diagnostic tool for hosts you manage, not a port scanner.

›Why can I connect locally but the checker says timeout?

Your service is listening but not reachable from the internet. Typical culprits: a cloud security group or firewall without an inbound rule for that port, the service bound to 127.0.0.1 instead of 0.0.0.0, or a router without port forwarding.

›Why are private IPs like 192.168.1.1 rejected?

The check runs from our servers on the public internet, where private ranges (10.x, 172.16-31.x, 192.168.x, 127.x) are not routable — and probing internal networks is blocked for security. Test private hosts from inside your own network instead.

Related tools

Check If a Port Is Open

Step-by-step port troubleshooting guide and tool.

Open Port Checker

Audit which of your service ports are exposed.

SSL Checker

Inspect the certificate served on port 443.