The connection is made from our server, so you see exactly what any computer on the Internet would see. Only TCP is checked, one port at a time and up to 5 times per minute; UDP cannot be verified reliably because an open UDP port is not required to respond.
How it works
A port is open when a program is listening on it and the connection can reach it through the router and any firewalls along the way. There is no reliable way to check this from your own network, because the test does not go through the router the way a connection from outside does. This tool attempts the connection from our server, which is on the Internet, and tells you how the target responded.
The result distinguishes three cases. Open means the connection was established. Closed means the device replied that nothing is listening on that port, so the connection gets through but the service is missing. No response means something silently dropped the connection, which is what firewalls, routers without a forwarding rule and connections behind CGNAT do.
Examples
203.0.113.25 : 25565SYN → SYN/ACK · 38 msOpen: a Minecraft server with port forwarding set up correctly, which other players can join.203.0.113.25 : 3389SYN → RST · 41 msClosed: the forwarding rule exists and reaches the computer, but Remote Desktop is disabled or listening on another port.203.0.113.25 : 8080SYN → (timeout 3 s)No response: the router has no rule for that port or a firewall is dropping the connection.Use cases
- Confirm that your router's port forwarding works before inviting others to a game server such as Minecraft, Valheim or Counter-Strike.
- Verify that an IP camera, a NAS, Plex or Home Assistant can be reached from outside your home.
- Make sure sensitive services such as Remote Desktop (3389), SMB (445) or a database are not accidentally exposed to the Internet.
- After a firewall change on a server, check that the service port still responds from outside.
- Find out whether your provider blocks a port, such as 25 for sending email, or whether your connection is behind CGNAT.
Frequently asked questions
Why does the port show as closed if I already opened it on the router?
Opening the port on the router only lets the connection through: a program also has to be listening. If the result is closed, the rule probably works and what is missing is the service, or the rule points to a local IP that no longer belongs to the computer. If the result is no response, check the computer's firewall and make sure the rule is saved and enabled.
What is CGNAT and why does it prevent opening ports?
With CGNAT the provider shares a single public IP among many customers and your router gets a private address, usually from the 100.64.0.0/10 range. Connections arriving at the public IP end up at the provider's equipment, not at your router, so no forwarding rule you configure can work. The solution is to ask your provider for a public IP of your own, or to use IPv6 or a tunnel.
Can I check UDP ports?
Not reliably. A UDP service only responds when it receives a message in its own protocol, so an open UDP port and a filtered one look the same from outside. For WireGuard, UDP game servers or VoIP, the practical approach is to test with the real client from another network, for example using your phone's mobile data.
Is checking a port the same as a port scan?
No. The tool opens a single TCP connection to the port you choose and closes it as soon as it is established, without sending any data. To prevent abuse, it accepts one port per query and up to 5 queries per minute, does not reach private or local addresses, and logs every check.
Why is my IPv4 address checked and not my IPv6 address?
Port forwarding is an IPv4 mechanism: with IPv6 every device has its own public address and what you configure is a firewall rule. If you have both, the tool suggests the IPv4 address, but you can choose the IPv6 address and check the device directly.
Is it dangerous to have open ports?
An open port exposes the service listening on it to the whole Internet, where bots constantly try passwords and known vulnerabilities. Open only what you need, keep the program up to date and avoid exposing Remote Desktop, SMB or databases: use a VPN for that instead.