Check whether a specific TCP port on a public host is open, closed, or unreachable.
A port can fail to respond for a few different reasons: nothing is actually listening on it, a firewall is blocking the connection, or the service is only bound to a private/internal interface rather than the public one. This tool can't distinguish between "blocked by firewall" and "nothing listening" - both show up the same way from the outside.
For security reasons, this tool refuses to check private, internal, or reserved IP ranges (like
192.168.x.x or 10.x.x.x) - only public hosts can be checked.
Known limitation: this check runs from Cloudflare's network, which cannot open connections back into Cloudflare's own network. If the host you're checking is itself served through Cloudflare (a huge share of the web is), you may see an incorrect "closed" result even when the port is genuinely open. This doesn't affect checks against hosts on other infrastructure.
Read more: Why Is My Port Closed? Checking Open Ports the Right Way.