Gmail using 6to4

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Tags: IPv6 6to4 gmail

I happen to look at an email destined to Gmail, and it turns out instead of 10.0.0.0/8 IPv4 addresses, they have now switched to IPv6 network block for 6to4 prefix (2002::/16) based on 10.0.0.0/8:

Received: by 2002:a25:83c5:0:0:0:0:0 with SMTP id YYYYYYYYYYYYYYYYYYYYYYYYYYYY;
        Mon, 7 May 2018 09:26:55 -0700 (PDT)
X-Received: by 2002:a50:b1b5:: with SMTP id XXXXXXXXXXXXXXXXXXXXXXXXXXX;
        Mon, 07 May 2018 09:26:55 -0700 (PDT)

I wonder why they do that, and maybe for migration (or communication with their native IPv6 space), they are probably running their own 6to4 anycast router (192.88.99.1).