Port Checker

Why the Check Has to Come From Outside

A port test is only meaningful when it is performed from a different network than the one the server sits on. From inside your own LAN, a port will look open even when your router never forwards it, and your firewall rules never apply to traffic that does not cross them.

Browsers cannot help here either. JavaScript cannot open an arbitrary TCP connection to an arbitrary port, which is why a checker that runs entirely in your browser cannot actually test anything. This tool opens a real TCP connection from UptyBots's infrastructure, so the answer reflects what any user on the internet would experience.

Open, Refused, Filtered: What the Result Means

A closed port is not a single condition. There are two very different ways for a connection to fail, and telling them apart is most of the diagnosis:

• Open – something accepted the connection. The service is running and reachable from the public internet.
• Refused – the host answered, but nothing is listening on that port. The machine is alive and routing works; the service is either stopped, crashed, or bound to a different address such as 127.0.0.1 instead of 0.0.0.0.
• Filtered – no answer at all, and the attempt timed out. Something is dropping the packets silently: a firewall, a security group, an ISP block, or a router that never forwarded the port.

The practical difference: "refused" points you at the service, "filtered" points you at the network. Chasing the wrong one wastes hours.

Common Ports Worth Testing

When a port number is well known, the result names the service so you can confirm you are looking at the right thing:

• 22 SSH, 80 HTTP, 443 HTTPS – the three most commonly checked after a server move or a firewall change.
• 25, 465, 587 SMTP – frequently blocked outbound by hosting providers and residential ISPs.
• 3306 MySQL, 5432 PostgreSQL, 6379 Redis – these should almost never be open to the internet. If this tool says they are, treat it as a finding, not a success.
• 25565 Minecraft, 27015 Source engine games, 30120 FiveM – the usual reason a game server does not show up in the public list. For these, the game server checkers go further and read back the server name, map and player count.
• 3389 RDP, 5900 VNC – remote desktop ports that are a standard target for automated attacks when exposed.

If the Port Should Be Open but Is Not

A practical order for narrowing it down, from the most common cause to the least:

• Check that the service is actually running and listening on all interfaces, not only on localhost. On Linux, ss -tlnp shows both the port and the address it is bound to.
• Check the host firewall – ufw, firewalld, or raw iptables rules on the machine itself.
• Check the provider firewall – AWS security groups, Hetzner and DigitalOcean cloud firewalls, and similar layers exist outside the machine and are easy to forget.
• Check port forwarding if the server is behind a home router, and confirm the internal IP has not changed since you set the rule.
• Confirm you are testing the right address – behind a proxy or a CDN, the public IP belongs to the proxy, and the origin port is not reachable directly by design.
• Confirm the protocol – this tool tests TCP. A service that only speaks UDP will always look closed here, which is normal rather than broken.

Frequently Asked Questions

Is this tool free?

Yes, completely free and without signup. For continuous checks with alerts when a port stops answering, a free UptyBots account lets you turn any host and port into a monitored target.

Can I check UDP ports?

Not with this tool. UDP has no handshake, so silence is ambiguous: an open UDP port and a firewalled one look identical from outside unless the service replies. UptyBots monitors do support UDP checks for game servers, where the service answers a specific query.

Can I scan a range of ports?

No. This is a single-port check, not a port scanner. Scanning ranges of ports on machines you do not own is unwelcome at best and illegal in some jurisdictions, so the tool deliberately tests one port at a time.

Why can I not check localhost or a private IP?

Because the check runs on our servers, not yours. 127.0.0.1 and addresses such as 192.168.x.x or 10.x.x.x would refer to our own network rather than to yours, so those requests are rejected outright.

The port is open. Should it be?

That depends entirely on the service. Web and game ports are meant to be public. Database, cache, and remote desktop ports usually are not, and finding them open from the outside is worth acting on immediately.

How often is the result cached?

For one minute per host and port. Repeating the same check immediately returns the same answer; wait a minute to get a fresh connection attempt.