What was the motivation for adding the IPV6_V6ONLY flag?

Posted by Jeremy Friesner on Stack Overflow See other posts from Stack Overflow or by Jeremy Friesner
Published on 2010-04-22T19:11:37Z Indexed on 2010/04/22 19:13 UTC
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IPv6

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setsockopt

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networking

In IPv6 networking, the IPV6_V6ONLY flag is used to ensure that a socket will only use IPv6, and in particular that IPv4-to-IPv6 mapping won't be used for that socket. On many OS's, the IPV6_V6ONLY is not set by default, but on some OS's (e.g. Windows 7), it is set by default.

My question is: What was the motivation for introducing this flag? Is there something about IPv4-to-IPv6 mapping that was causing problems, and thus people needed a way to disable it? It would seem to me that if someone didn't want to use IPv4-to-IPv6 mapping, they could simply not specify a IPv4-mapped IPv6 address. What am I missing here?

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