I'm trying to add a IPv6 address to the security group of an EC2 instance, but I need to specify it as a CIDR block. It's a full IPv6 address, like this fake address: 2600:1003:c10c:b42c:1956:fed1:db13:ff42. How can I convert this to a CIDR block/range?
As vcsjones said, AWS security groups do not support IPv6 - primarily because EC2-VPC does not support IPv6.
See this thread on the AWS forums for some relevant info.
This thread has more info on Amazon's lack of support for IPv6 on EC2.