How to read the result
Open means the TCP handshake completed: something listens on that port and the network lets us reach it. We do not send any data after the handshake.
Closed covers two cases. A refused connection means the host is reachable but nothing listens on the port. No answer at all within 10 seconds usually means a firewall drops the packets.
Type a name or an IP address, then the port. A port can be open from your office and closed from the internet: that is often what a firewall is for, and why this tool tests from outside.
Why a port is closed
- The service is stopped, or listens on 127.0.0.1 only instead of all interfaces.
- A firewall blocks it: the host firewall, a cloud security group, or the hosting provider.
- At home, the router has no port forwarding rule to the machine.
- The name points to another machine than the one you configured.
Common ports
22 is SSH, 25 and 587 are mail, 80 and 443 are HTTP and HTTPS, 3306 is MySQL, 5432 is PostgreSQL and 6379 is Redis. A database port open to the whole internet is usually a mistake: restrict it to the machines that need it, and use this tool to confirm it is closed.
Questions
Can I check a UDP port?
Not here: this tool tests TCP. SolidPing has a UDP check, and checks for DNS, NTP, SIP and other UDP services, once you sign up.
Can I check a private address?
No. Private, local and reserved addresses are refused. To watch an internal service, run a SolidPing private agent inside your network.
How do I know when the port closes?
Add a TCP check in SolidPing. It connects every minute and alerts you when the port stops answering.