The http://whatsmyrouterip.com/ website can be used to resolve the public internet WAN IP address of a gateway router. However, it also shows the the private lan IP address of the gateway router and the private lan address of the device sending the request (e.g laptop).
How does the site get access to the private lan IP addresses given that the NAT source IP address of the page request is that of the gateway router? HTML headers?
E.G. Gateway router public IP address: 2188.8.131.52. Gateway router private LAN address 192.168.1.1. Laptop private LAN address 192.168.1.25.
The site is HTTP so all this information is submitted in clear text.