There&#39;s a few updates to original RFC regarding choice of transport layer for connections.  It used to be pretty much prefer IPv6 but in some systems that doesn&#39;t make sense. ... routing or address availability...which is why other things such as happy eyeballs came along<br>
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RFC 6724 makes more sense in this regard<br>
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Say,  for example,  my server only has IPv6 connectivity to the Internet and a local 192.168.x.x address (with no NAT) . A lookup of a remote server <a href="http://xyz.com">xyz.com</a> gives me back an A and AAAA record.  Will you be trying to use that v4 result and leave the server non contactable?  I&#39;d hope not<br>
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(and quite a few people are now having such servers as IPv6 only....)<br>
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alan<br>
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Sent from my Android device with K-9 Mail. Please excuse my brevity.