No one apparently noticed until today that Google's #Gmail infrastructure appears to be using an #IPv6 transition mechanism 6to4 internally, and in a way that is technically in violation of spec. I realized they are mapping IPv4 10/8 addresses (see IETF RFC 3056).
As Brian later points out in the thread, this is odd, but who cares?
https://mailarchive.ietf.org/arch/msg/v6ops/wkvXhL8Z_Jy9fFFki_T2VnFi958/
I find it kind of fascinating that this has been going on for years. Operationally curious and and presuming Google has it's own internal 6to4 gateways for this and maybe other things. From a research and security perspective, I'm wondering if a collection of emails can be analyzed to derive any kind of patterns or structure. Practically I'll ask some Google people I know when I see them next and see if they'll just tell me more.