https://tomchen.github.io/symmetric-nat-test/
Symmetric NAT Test
Credit https://webrtchacks.com/symmetric-nat/
Credit https://github.com/tomchen/symmetric-nat-test
Go to the “Symmetric NAT Test” page (https://tomchen.github.io/symmetric-nat-test/) and see if you are behind a Symmetric NAT or Normal NAT (Full-cone NAT or other non-symmetric)