Port checker
Test whether a TCP port (SMTP, POP3, FTP, …) is open on a host.
Services listen on TCP ports — mail on 25/587, web on 80/443, FTP on 21, and so on. Enter a host and port to confirm the service is reachable and accepting connections. Private and internal addresses are blocked for safety.
Frequently asked questions
Open vs closed — what does it tell me?
Open means a service accepted the connection. Closed or filtered means nothing is listening, or a firewall is blocking it.
Which ports are common?
25/465/587 for mail submission, 110/995 POP3, 143/993 IMAP, 21 FTP, 22 SSH, 80/443 web.
A port is open locally but shows closed here — why?
A firewall, security group, or NAT between the internet and your host is likely blocking external access. The service is listening, but only on an interface or from sources our checker can't reach.
Does an open port mean I'm insecure?
Not on its own — open ports are how services work. The risk is exposing ports you don't need (like databases or admin panels) to the public internet; those should be firewalled to known sources.
Why can't I check a private IP or localhost?
Loopback, private (10/8, 172.16/12, 192.168/16), and cloud-metadata addresses are blocked so the tool can't be used to probe internal networks. Test those from inside the network itself.
How is this different from the port scanner?
This checks one specific port you name; the port scanner probes a curated list of common ports at once to show your overall exposure. For ongoing watching, SJ Monitor's port monitoring alerts you the moment a port stops responding.
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