How do Internet-Of-Things apps detect if the device is local or not?

_x000D_ _x000D_ Among other devices, I have a Belkin WeMo switch and a 3M WiFi Thermostat. Both come with Android apps. When I open the app when at home (phone on the same WiFi subnet as the devices) the app says the devices are "local", or that the app is connected "directly" to the device. Any change I make is then directly reflected on the device. If I connect from anywhere outside my house, it some how "knows" that the device is not local. If I understand correctly, it then updates its own server somewhere, and eventually the device at home gets the update. How does an app do that? I want to replicate that functionality. Basically, I have a Raspberry Pi that, let's say does something over GPIO and I can trigger that action by visiting a web page. On my home network, the web page is accessible at http://192.168.10.25/do_something ...Outside my home network, the web page is accessible at http://my_custom_name.some_free_dynamic_dns.com:8080/do_something. I have set up my router to port forward 8080 on WAN to 192.168.10.25:80 on LAN. This works. How do I avoid having 2 pages? I want to write an app which effectively shows me the 192.168.10.25/do_something URL. When I am at home, it should "automatically" detect that the phone is on the same network as the URL, and just open 192.168.10.25/do_something directly ... and when I am outside, it should "automatically" detect that too and launch http://my_custom_name.some_Free_dynamaic_dns.com:8080/do_something. The key part here is that when at home, even if my ISP is down and host names aren't resolving, it shouldn't matter because everything is local. How do I go about doing this?