What Is a Port Checker?
A port checker tests whether a specific TCP port on a remote host is open and accessible from the internet. It works by attempting to establish a TCP connection — if it succeeds, the port is open; if it's refused, the port is closed; if there's no response, the port is likely filtered by a firewall.
Why Would a Port Be Closed?
Ports can be closed for several reasons: no service is running on that port, a software firewall on the server is blocking it, or a network-level firewall (such as a cloud provider's security group) is dropping traffic before it reaches the server.
Common Use Cases
- Web servers: Check that ports 80 (HTTP) and 443 (HTTPS) are publicly accessible after deployment.
- SSH access: Verify port 22 is open if you're trying to connect remotely and the connection is failing.
- Email servers: Diagnose SMTP (25), IMAP (143), or POP3 (110) delivery issues.
- Game servers: Confirm your game server ports are reachable by players.
- Security audits: Check which ports are publicly exposed on your own infrastructure.
Filtered vs Closed: What's the Difference?
A closed port actively sends back a TCP RST (reset) packet — the host is reachable but no service is listening. A filtered port produces no response at all, usually because a firewall is silently dropping packets. From a security perspective, filtered ports are often preferable — they reveal less about your server's configuration.