user859: Have you heard about support for IPv6 protocol for any Ethernet shield?
As far as I know only IPv4 is supported.
Maybe some one would like to write library?

user220: There is no library, and there physically cannot be one. IPv4 is built into the Ethernet shield itself. (It has a Wiznet 510 chip.)

Shields based on the MicroChip ENC28J60 chip (such as those sold by Ekitszone and Nuelectronics) are suitable for the implementation of an IPv6 or dual IPv4/IPv6 stack. This approach requires the TCP state machine to be implemented in software and results in more complex arduino code.

Source: this tutorial. (You may find it useful, I'd recommenced reading it.)
There is this library for IPv6 on other hardware, not for the official Ethernet shield.

user24470: I have been working on an IPv6 library for Arduino, called EtherSia, that works with the ENC28J60 chip, but with a lower overhead compared with libraries that use Contiki / uIP. It is comparable to the EtherCard library for IPv4.
https://github.com/njh/ethersia
While the Wiznet chips do not support IPv6 natively, they do have a MACRAW mode that allows you to send Layer 2 Ethernet frames directly. I have managed to get IPv6 working with the Wiznet W5100 and W5500 chips, which is what the official Arduino Ethernet Shield and Ethernet Shield 2 use.
So it is possible...

859: <p>Have you heard about support for IPv6 protocol for any Ethernet shield?
As far as I know only IPv4 is supported.
Maybe some one would like to write library?</p>
 220: <p><strong>There is no library, and there physically cannot be one.</strong> IPv4 is built into the Ethernet shield itself. (It has a <em>Wiznet 510</em> chip.)</p>

<blockquote>
  <p>Shields based on the MicroChip ENC28J60 chip (such as those sold by Ekitszone and Nuelectronics) are suitable for the implementation of an IPv6 or dual IPv4/IPv6 stack. This approach requires the TCP state machine to be implemented in software and results in more complex arduino code.</p>
</blockquote>

<p><em>Source: <a href="https://www.adafruit.com/blog/2012/09/17/ask-an-educator-how-can-i-get-my-arduino-to-use-ipv6/" rel="nofollow">this tutorial</a>. (You may find it useful, I'd recommenced reading it.)</em></p>

<p>There is <a href="https://sites.google.com/site/ghoelzl/ipv6ethershield" rel="nofollow">this library for IPv6 on <em>other hardware</em>, not for the official Ethernet shield</a>.</p>
 24470: <p>I have been working on an IPv6 library for Arduino, called EtherSia, that works with the ENC28J60 chip, but with a lower overhead compared with libraries that use Contiki / uIP. It is comparable to the EtherCard library for IPv4.</p>

<p><a href="https://github.com/njh/ethersia" rel="nofollow noreferrer">https://github.com/njh/ethersia</a></p>

<p>While the Wiznet chips do not support IPv6 natively, they do have a MACRAW mode that allows you to send Layer 2 Ethernet frames directly. I have managed to get IPv6 working with the Wiznet W5100 and W5500 chips, which is what the official Arduino Ethernet Shield and Ethernet Shield 2 use.</p>

<p>So it is possible...</p>
