If you have a server with a public address, a machine with a private address can simply use e.g. curl to access some (even non-existant) url on the server perio

If you have a server with a public address, a machine with a private address can simply use e.g. curl to access some (even non-existant) url on the server periodically which can then be extracted from the logs by a script on the server. I actually use this to announce my internal address, even without a web server, just by accessing a locked port on the server, where the firewall logs this access.