This assumes you are using the current version of the operating system (iOS 10.2.1 as of this post)
What to do:
- On the home screen, tap Settings.
- Select WiFi to open a list of currently available wireless networks.
- Locate the wireless network to which you are currently connected in the list, then tap on the blue circle with a white arrow that appears to the right of the network name.
- The IP Address field should be the first field listed under the DHCP tab.