AWS 的 VPC 以前都只能設定 public IPv6 address，現在總算是可以設定 private IPv6 address 了：「AWS announces private IPv6 addressing for VPCs and subnets」，另外一篇講的比較詳細：「Understanding IPv6 addressing on AWS and designing a scalable addressing plan」。
IPv6 address 的熟悉度沒有 IPv4 address 高... 看起來 AWS 所提到的 private 包括了 GUA 與 ULA：
Private IPv6 addresses on AWS can include ULA, which are intended for local communication, and private GUA, which could be globally routable according to protocol definition.
GUA 好像是就是拿 public ip 區段用 (反正 IPv6 區段夠大)？反而是 ULA 這邊用的 fd00::/8 跟以前 IPv4 的 private range 剛好相對應，比較好理解...