You have a router and many devices can connect to it to access the internet. From what I've read, the router assigns private interal ip addresses to the devices that are connect so it knows how to direct the traffic. The router however, is assigned one ip address by the ISP. What the world sees from any request coming from that router, whether from a desktop or a tablet, is that one ip address of the router. So how can a server tell how many users on the other end of that ip address? I know devices have their unique mac addresses, but I read that that information isn't sent to servers.
A more concrete scenario would be for video hosting websites like youtube. How would it know how many users are using the same ip? (for view counts) Also, in places where you expect many users using the same "wifi" such as Starbucks, Burger King, or some university, how are devices connected differentiated on the web?