At WWDC 2015 we announced the transition to IPv6-only network services in iOS 9. Starting June 1, 2016 all apps submitted to the App Store must support IPv6-only networking. Most apps will not require any changes because IPv6 is already supported by NSURLSession and CFNetwork APIs.
If your app uses IPv4-specific APIs or hard-coded IP addresses, you will need to make some changes. You can learn how to make the necessary changes by reading this support document and watching the 'Your App and Next Generation Networks' video.