IPv6 on mobile networks, was Update to BCP-38?

[johnl at iecc.com (John R. Levine)]

In article <d3f78384-9b25-c4c4-495f-5dcc0e0c1925 at satchell.net>,
Stephen Satchell <nanog at nanog.org> wrote:
> My AT&T cell phone has both IPv4 and IPv6 addresses.  The IPv4 address
> is from my access point; the IPv6 address appears to be a public address.

My AT&T cellphone (via MVNO Tracfone) has a 10/8 IPv4 address and IPv6 
address 2600:380:28be:8b34:2504:2096:6ac7:6262.  But when I connect to a 
web site that reports the connecting address, it says I'm 
2600:387:a:9a2::8.

What's going on there?  Those are both within at&t's 2600:300::/24', but 
am I behind a NAT66?  An aggressive web cache?

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