Port Checker

Open, closed, or filtered?

A port is only open if a process is listening and the network path lets us reach it. This checker attempts a TCP connect from our infrastructure, so the result is what the public internet sees — not what localhost sees on your laptop.

• Open: the handshake completed. Something accepted the connection.
• Closed: the host is reachable but sent a RST — nothing is listening on that port.
• Filtered / timeout: no decisive reply. Often a firewall or security group, but also an offline host or a bad route.

Frequently asked questions

How do I check if a port is open?

Enter a hostname and TCP port above. We attempt a connection from our servers. Open means something accepted the handshake. Closed (connection refused) means the host is reachable but nothing is listening. A timeout usually means a firewall is filtering the port, or the host is unreachable.

What is the difference between open, closed, and filtered?

Open: the TCP handshake completed. Closed: the host sent a RST (connection refused). Filtered or timeout: no decisive reply — often a firewall, but also routing failure or an offline host.

Can I check port 22, 3306, or 5432?

Yes. Any TCP port from 1 to 65535. Common checks are 443 (HTTPS), 80 (HTTP), 22 (SSH), 3306 (MySQL), 5432 (PostgreSQL), and 6379 (Redis). We do not check private or internal addresses.

Why does a port look closed from here but open on my machine?

Your laptop is on a different network path. A process listening on localhost is not reachable from the internet. A cloud firewall, security group, or ISP filter can also allow your office IP and block ours.

How do I monitor a port continuously?

A one-off check only tells you about right now. Webalert TCP monitors a host and port every few minutes and alerts you by email or Slack when it stops accepting connections. Free to start, no credit card.

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