Apple has announced on its developer blog that it will require all apps submitted to the iOS App Store to support IPv6-only networks from June 1 2016. Apple originally introduced the transition a year ago at WWDC 2015, implying it would soon become a requirement.
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A small subset of apps in the App Store will need to update their code to support IPv6-only networks; Apple has posted relevant guidelines on its developer documentation website. Most developers will not be affected as most apps do not have a need to use a custom networking stack and rely on the system networking APIs. Obviously, Apple makes sure the system frameworks are up-to-date and already support IPv6-only networks.
IPv6 is an upcoming network standard that is meant to supersede IPv4, as the latter is running out of available resource addresses. Eventually, IPv6 will replace IPv4 completely. Apple needs to make sure its platforms are ready in advance of the technology ecosystem shift.
June 1st is also the cut-off date for another new developer requirement: all Watch apps must use the native watchOS 2 WatchKit APIs from June 1st.