In the App Review, they test the app on a IPv6 only network, if they have any network-related error, the app will be rejected.
"We will be reviewing on an IPv6 network, so if your app isn’t compatible with the IPv6 addressing, it may fail during review".
> At WWDC 2015 we announced that iOS 9 will support IPv6-only network services. All apps submitted to the App Store must support IPv6 starting in early 2016.