Port scanner
Check which common ports are open, closed, or filtered on a host.
This online port scanner probes a host across a fixed set of common ports and reports which are open, closed, or filtered. Open ports reveal the services a machine exposes to the internet, so the result helps you confirm intended services are reachable and spot ones that should not be listening at all. An unexpected open port can signal a misconfiguration or a security exposure worth closing. The scan runs from our network against a curated common-port list (not arbitrary ranges) with private and internal addresses blocked.
Frequently asked questions
What's the difference between closed and filtered?
A closed port actively refuses the connection, while a filtered port silently drops it, usually due to a firewall. Filtered results mean a firewall is shaping what the scanner can see.
Which ports should be open on my server?
Only the ones your services genuinely need, such as 443 for HTTPS, with everything else closed or firewalled. Unexpected open ports deserve investigation.
Why only common ports, not a full range?
A full 65,535-port sweep is slow and is what attackers do — this tool checks the handful of ports that actually matter for most services, quickly and responsibly.
Can you alert me when a port changes state?
Yes. SJ Monitor offers continuous port monitoring that checks your chosen port and notifies you the moment it opens, closes, or stops responding.
Can I scan any host I want?
Only scan hosts you own or have permission to test. Private, loopback, and cloud-metadata addresses are blocked, and abuse is rate-limited.
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