A short IPv6 story
A little while ago, we leased a new dedicated server for our business. In the spirit of keeping up with technology and looking to the future, I got one that had ipv6 connectivity.
Shortly after migrating to this new server I was made aware that an application that uses Google Cloud Messaging was no longer working properly, Google was reporting that the server was unauthorised to send out messages. I checked everything and couldn't see the problem, the code was exactly the same as running on the older server and the new ipv4 address had been entered correctly into the Google control panel.
Only after days of digging did the cause of the problem come to light. The cURL library used to send out messages was automatically defaulting to ipv6 rather than ipv4 and so Google was rejecting the messages since the ipv6 address wasn't authorised. Fine - I'd just enter that into the Google control panel then... but wait, what's this? Google's control panel only accepts ipv4 addresses...
Long story short - I disabled ipv6 on the new server as the simplest solution with the added benefit that no more stuff would break for no apparent reason.