detecting-nat64-prefix.html

---
url: /2015/12/detecting-nat64-prefix/
title: "Detecting NAT64 Prefix"
date: "2015-12-01T09:46:00.000+01:00"
tags: [ IPv6,DNS,NAT ]
---

<p>If you’re a host running on an IPv6-only network, you might want to detect the IPv6 prefix used for <ahref="https://my.ipspace.net/bin/get/IPv6E101/NAT64%20and%20DNS64%20in%2030%20minutes.pdf?doccode=IPv6E101">NAT64</a> (for example, to transform IPv4 literals a clueless idiot embedded into a URL into IPv6 addresses).</p>
<pclass="info">Apple has a <ahref="https://developer.apple.com/library/prerelease/ios/documentation/NetworkingInternetWeb/Conceptual/NetworkingOverview/UnderstandingandPreparingfortheIPv6Transition/UnderstandingandPreparingfortheIPv6Transition.html">wonderful developer-focused page</a> describing NAT64 and DNS64, including the way they synthesize IPv6 addresses from IPv4 literals. You (<ahref="https://tools.ietf.org/html/rfc6919">RFC 6919</a>) MUST read it.<!--more--></p>
<p>There’s a standard way to do that: do a AAAA DNS lookup for ipv4only.arpa, which always maps into 192.0.0.170/171, and extract the NAT64 prefix from the response.</p>
<p>For more details, read <ahref="https://tools.ietf.org/rfc/rfc7050">RFC 7050</a> (Discovery of the IPv6 Prefix Used for IPv6 Address Synthesis).</p>
<pclass="more">Hat tip to Gert Doering who pointed out <em>ipv4only.arpa</em> on one of the IPv6 mailing lists.</p>
<h4>More details?</h4><p>Sure. You’ll find more about NAT64 in <ahref="http://www.ipspace.net/IPv6_Transition_Mechanisms">IPv6 Transition Mechanisms</a> webinar, and a wealth of IPv6-related information on the <ahref="http://www.ipspace.net/IPv6">ipSpace.net IPv6 page</a>.</p>