As we've been seeing a number of emails recently on this from people getting their apps rejected due to the IPv6 requirements Apple put in place earlier in the year, I'm putting up some resources for those who might need it. Here are some of Apple's own resources on things to do for your app to conform to the submission requirements.
Properly put, the Apple requirement is that an app must be able to work on an IPv6-only network. This means no usage of IPv4-only APIs, no IPv4 literals, and avoidance of some pre-flight checks. It must be able to work in a NAT64 environment. You do not need IPv6 support on the server-side yet. If they required that now, people hosting on AWS or GAE would be in trouble. However those services would then have more calls to finish up their implementations. That said, it's still a good idea to work towards server-side support.Apple's Tech Note on IPv6 networks, and app requirementsWWDC 2015 Video on the same, and the NAT64 implementation in El Capitan