Merge pull request open-guides#97 from tomschlick/patch-1

document IPv4 & IPv6 access to S3

README.md

@@ -639,6 +639,7 @@ S3
 	-	[ObjectiveFS](https://objectivefs.com/) ([discussion](https://news.ycombinator.com/item?id=10117506)) is a commercial solution that supports filesystem features and concurrent clients.
 -	If you are primarily using a VPC, consider setting up a [VPC Endpoint](http://docs.aws.amazon.com/AmazonVPC/latest/UserGuide/vpc-endpoints.html) for S3 in order to allow your VPC-hosted resources to easily access it without the need for extra network configuration or hops.
 -	**Cross-region replication:** S3 has [a feature](https://docs.aws.amazon.com/AmazonS3/latest/dev/crr.html) for replicating a bucket between one region and a another. Note that S3 is already highly replicated within one region, so usually this isn’t necessary for durability, but it could be useful for compliance (geographically distributed data storage), lower latency, or as a strategy to reduce region-to-region bandwidth costs by mirroring heavily used data in a second region.
+-	**IPv4 vs IPv6:** For a long time S3 only supported IPv4 at the default endpoint `https://BUCKET.s3.amazonaws.com`. However, [as of Aug 11, 2016](https://aws.amazon.com/blogs/aws/now-available-ipv6-support-for-amazon-s3/) it now supports both IPv4 & IPv6! To use both, you have to [enable dualstack](http://docs.aws.amazon.com/AmazonS3/latest/dev/dual-stack-endpoints.html) either in your preferred API client or by directly using this url scheme `https://BUCKET.s3.dualstack.REGION.amazonaws.com`.
 
 ### S3 Gotchas and Limitations