Discovery of external IP address (Noip.com)

[sindy]

I was also under the impression that the DNS name in ip/cloud is not accessible when behind a NAT modem (and needs port forwarding) or are you saying that this MT dns name will me sufficient to enter it on the remote side MT and through this discovery be able to create a successful link?

Looking for ways for each device MT (A) and MT (B) to know each other’s Internet address so an L2TP/Ipsec link can be established …

As others have written: you don’t need any special measures to learn the public address behind which your router is NATed, but knowing that address is not sufficient to make an L2TP server listening at your router accessible from the internet.

So one way to establish a VPN between two routers is to establish a VPN tunnel from both to a third router on a public IP.

Another way is to use an IKEv2 IPsec site-to-site tunnel where both routers act as initiators, but it is only possible if the NAT devices between each of your two routers and the internet keep the source UDP port unchanged whenever possible. See more here.