I\'ve submitted my app in AppStore, they reject if due to following reason.
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At WWDC 2015, apple announced the transition to IPV6-only network services in iOS 9. Now from June 1, 2016 apple has made mandatory that the app should support iPV6.
If developers use the standard networking APIs like NSURLSession and CFNetwork APIs provided by apple, they should not experience any problem. Otherwise you will have to move your code to the APIs that support IPV6.
If you are looking for to use some standard network library like AFNetworking. According to this site AFNetworking 3.0 support IPV6.
Apple has a documentation on how to test IPV6 support.