Why the site exists
CGNAT Check is an independent utility focused on one troubleshooting problem: understanding why a home connection cannot accept expected inbound traffic. Port-forwarding problems are often described as a single router-setting issue, even though the real cause may be CGNAT, double NAT, the wrong WAN value, a local firewall, an inactive service, or a connection model that never used direct inbound traffic. The site keeps those related questions in one narrowly focused toolkit.
The aim is not to diagnose every home-network problem. It is to help a reader gather better evidence, distinguish common NAT topologies, and ask an ISP or equipment vendor a more precise question.
How the checker is interpreted
The checker compares the public IPv4 visible to the web server with the WAN IPv4 that the user manually reads from a router or gateway. Address ranges and exact public-IP matches provide useful signals, but the result remains best-effort because the browser cannot inspect every router or provider system in the path.
The complete methodology page documents how shared, private, matching, and mismatched address cases are handled and what the checker deliberately does not claim.
Worked examples are either clearly labelled illustrations or documented observations from development troubleshooting. For example, the mobile-hotspot guide records the exact laptop and gateway values observed in one Android tethering test and separates what those values established from what remained unknown.
