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copied!<p>On POSIX systems you can use <code>inet_pton</code> and <code>inet_ntop</code> in combination to do canonicalization. You will still have to do your own CIDR parsing. Fortunately, I believe the only valid CIDR syntax for IPv6 is the /number_of_bits notation, so that's fairly easy.</p> <p>The other issue you will run into is the lack of support for interface specifications. For link-local addresses, you will see things like <code>%eth0</code> on the end to specify what link they are local too. <code>getaddrinfo</code> will parse that but <code>inet_pton</code> won't.</p> <p>One strategy you could go for is using <code>getaddrinfo</code> to parse and <code>inet_ntop</code> to canonicalize.</p> <p><code>getaddrinfo</code> is available for Windows. <code>inet_pton</code> and <code>inet_ntop</code> aren't. Fortunately, it isn't too hard to write code to produce a canonical form IPv6 address. It will require two passes though because the rule for 0 compression is the biggest string of 0s that occurs first. Also IPv4 form (i.e. <code>::127.0.0.1</code>) is only used for <code>::IPv4</code> or <code>::ffff:IPv4</code>.</p> <p>I have no Windows machine to test with, but from the documentation it appears that Python on Windows supports <code>inet_pton</code> and <code>inet_ntop</code> in its socket module.</p> <p>Writing your own routine for producing a canonical form might not be a bad idea, since even if your canonical form isn't the same as everybody else's, as long as it's valid other people can parse it. But I would under no circumstances write a routine of your own to <strong>parse</strong> IPv6 addresses.</p> <p>My advice above is good for Python, C, and C++. I know little or nothing about how to solve this problem in Java or Javascript.</p> <p><strong>EDIT</strong>: I have been examining getaddrinfo and its counterpart, getnameinfo. These are in almost all ways better than <code>inet_pton</code> and <code>inet_ntop</code>. They are thread safe, and you can pass them options (<code>AI_NUMERICHOST</code> in <code>getaddrinfo</code>'s case, and <code>NI_NUMERCHOST</code> in <code>getnameinfo</code>'s case) to keep them from doing any kind of DNS queries. Their interface is a little complex and reminds me of an ugly Windows interface in some respects, but it's fairly easy to figure out what options to pass to get what you want. I heartily recommend them both.</p>

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