Gns3 external ip finder

I am using software called gns3 to build networks.

I have the need to find out devices external ip from within an gns3 internal network. This would be like a pc behind a router doing NAT.

Please could someone tell me if there is a way that you can echo out the clients external ip in php so I can add a web server to one of my virtual networks within gns3 so I can visit it on some devices and find their external ip’s for testing?

Note that my gns3 networks are not connected to the internet so can’t connect externally.

Thanks in advance

Haven’t done php in 10+ years, but something like:

<? $_SERVER['REMOTE_ADDR'] ?>

?

You could probably use one of the free what is my ip websites