Offene Ports prüfen

What the results mean

Open – something accepted the connection. Your port forwarding and firewall are working.
Closed – the host answered, but nothing is listening on that port. Check that the service is running and bound to the right address.
Filtered – no reply at all. A firewall is dropping traffic, port forwarding is missing, or your ISP blocks the port.

Give the server a fixed local IP address, using a DHCP reservation.
Forward the external port to that IP and port on your router.
Allow the port in the server’s own firewall.
Check that your ISP gives you a public IPv4 address. If the router’s WAN address starts with 100.64–100.127, you are behind carrier‑grade NAT and forwarding will not work over IPv4.
Test from outside your network – this tool does exactly that.

Frequently asked questions

Does this check UDP ports?

No, only TCP. UDP has no handshake, so an open UDP port cannot be confirmed reliably from outside.

Which ports do ISPs commonly block?

Port 25 (SMTP) is blocked on most residential lines. Some ISPs also block 80 and 443 on home connections.

My IP address changes. How do I keep a stable address?

Use dynamic DNS. A uk.app name updates automatically when your IP changes – see the home server guide.