Custom port scan

When to use it

• Checking that port forwarding on a router works — for a camera recorder, a game server or remote access.
• Confirming that a firewall rule you just added (or removed) took effect.
• Testing a service on a non-standard port, such as a web panel on 8443.

The test uses TCP. UDP services — WireGuard (51820), OpenVPN in UDP mode, most VoIP/SIP — can’t be checked this way.

Port shows closed? Check, in order

1. The service is running and listening on all interfaces, not only on 127.0.0.1.
2. The computer’s own firewall (Windows Defender Firewall, ufw, firewalld) allows the port.
3. The router forwards the port to the right internal IP address — and that device’s IP hasn’t changed.
4. Your connection has a real public IP: if the router’s WAN address starts with 100.64–100.127 or is a private address, the provider uses carrier-grade NAT and incoming connections can’t reach you.
5. Some providers block certain ports on home lines, port 25 in particular.