At WWDC 2015 Apple 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.
Learn how to ensure compatibility by reading Supporting IPv6 DNS64/NAT64 Networks.
Source - Apple Developer News