When your iPhone connects to Wi-Fi, it's assigned a local IP address by the router that controls the network to which you are connected. If you need to find the local IP address assigned to your iPhone go to Settings and select Wi-Fi (should be just below Airplane Mode).
Once selected, you should see the Wi-Fi network to which you are connected as well as other networks if they are available. Tap the information icon to the right of the network you are connected. It's the lowercase i in the circle.
Your local IP address will be labeled as IP Address. In this case, my iPhone's IP Address is 192.168.1.13