There are two IP address assigned to each device on a Wi-Fi network.
An Internal (Private) and an External (Public) IP address.
Internal IP address is used for communication between Your computer and a smartphone or whatever devices that are connected to that Wifi network. This is unique for each. If they are same the system cannot differentiate between each device.
While, the External IP address is that visible to the the internet - the websites, the internet connected Apps so on. By default, Routers with NAT (all routers today have it) assign same Public IP address to all devices under its network.