Yes, it's IPv6.
I'm currently having my network experimentally run IPv6-only (For some devices).
In order to make this work, I've set up a DNS64 and NAT64 server on my router, so that even sites without IPv6 can be contacted using IPv6.
When I monitor the network traffic without the NAT64 (And thus using IPv4 again), I will get the URLs for specific sites.
When I enable NAT64 and thus the traffic is IPv6, the URLs for the same site are missed.
But don't take this special setup, the problems reported here:https://www.donationcoder.com/forum/index.php?topic=41297.0
are directly related to it too.
URLSnooper will no longer catch URLs for YouTube, Google Video, ... due to the lack of IPv6 monitoring as soon as the user's site is able to handle IPv6 and thus contacts these sites using IPv6.
And while URLSnooper does NOT catch the URLs, this utility which does about the very same but also for IPv6http://www.nirsoft.net/utils/http_network_sniffer.html
DOES catch them.
It just doesn't have a filter for multimedia URLs which makes digging for the right URLs a PITA.