For a long while now, a technical issue with the Project Gutenberg
   website has been making it completely inaccessible to a large number of
   Internet users. The only people that can access the site are people
   that either have an IPv4 address *only* or have a *native* IPv6
   address. Anyone using an IPv6 tunnel cannot connect to the website.
   Many ISPs automatically configure customer equipment to use an IPv6
   tunnel, so there are large swathes of people using IPv6 tunnels who
   have never heard of IPv6 and don’t know how to turn it off or use a
   VPN. To these people, your site has appeared to down for months/years,
   and it does not appear to be a DNS problem.


   Please stop filtering ICMPv6 traffic, so that more people can connect,
   access your ebooks, and possibly even donate or volunteer.


   An IPv6 tunnel is an intermediary gateway server that allows
   IPv4-equipped machines to be assigned IPv6 addresses and thus connect
   to the IPv6 portion of the Internet.


   - https://en.wikipedia.org/wiki/IPv6

   - https://www.google.com/intl/en/ipv6/

   - http://www.worldipv6launch.org/

   - Forum thread I started to figure out this issue with Project
   Gutenberg:
   https://www.reddit.com/r/ipv6/comments/4v8ncz/project_gutenberg_not_wor
   king_over_ipv6/