title: What was the motivation for adding the IPV6_V6ONLY flag?
body: <p>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.</p>
  
  <p>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?</p>