I have an internet connection at my residence through my cable provider. Obviously it hooks into my cable modem which them hooks into my router where the signal is switched to the wireless access point and transmitted to my desktop.
If I wanted to communicate to another device on my network, wired or otherwise, I’d use the IP of the network chip on board that device, and the switch would take care of the difference. If I wanted to connect to a server on the internet, I’d use the public IP of the modem that server is communicating through – or more likely, the place holder text that gets converted to that IP.
But what if I wanted to communicate across modems (from the one I own to say, one at my place of employment, or to my neighbors modem down the road), to a specific device in a group of devices communicating to their service provider through a single modem?
If you want your internal network device’s to be able to talk to another distinctly separate network’s internal devices across the public internet, you most likely need a VPN connection of some sort… either a soft client on your PC to a VPN server on the remote end or a hardware VPN (router to router), this would give you network access to devices on the remote network.
Depending on your application, you may be able to use something like TeamViewer to take control of a remote machine to do what you need as well, it is generally less difficult to implement but make sure to check with the administrator for the remote network on their security policies.