1. Confirm a service is listening
Check the application's configured TCP port on the computer hosting it. Confirm the process is running and bound to the intended interface. If another device on the same local network cannot connect, solve that local service or host-firewall issue before investigating internet forwarding.
2. Compare the public address
Open What Is My IP from the connection you intend to test. A VPN or proxy can make the visible address belong to another network. The checker deliberately limits targets to the visitor's own public IP; it is not an arbitrary-host scanning service.
3. Check the network path
If your router's internet-facing address differs from the public address, an upstream router, VPN or provider NAT may be involved. The shared range 100.64.0.0/10 is used for carrier-grade NAT. A forwarding rule on your own router cannot by itself create a mapping through a provider's NAT. Ask the provider what inbound connectivity is available.
4. Review targeted access rules
Where you control the public router, verify the external TCP port, internal destination and service port. Review the host firewall too. Avoid disabling the whole firewall or exposing an administration service merely to get an open result. Use the application's documented access method.
Confirm the result
Run Open Port Checker again while the service is listening. An open TCP connection proves reachability at that time, not application correctness, authentication strength or UDP availability.
Sources
Reviewed by NosHost on 26 October 2026. Shared address space for carrier-grade NAT · NosHost checker scope.