And 40% * 40% = 16% if you're doing P2P.

IPv6 is an option, but not the one true answer (yet?). It helps sometimes.

For Tailscale, IPv6 helps about 7% of the time to bust a NAT connection.

Sad. Where's that other 16-7=9% go? Firewalls? Or do Tailscale's users have even lower adoption of IPv6?

The 7% is how often it ends up helping after all the traditional IPv4 traversal tricks.

We race all the options and see which works & is fastest. It's likely it's more like 16% or more (I don't have that number handy) in practice but 7% is just how often it would've not worked at all and had to go to DERP relays if it weren't for IPv6 being an option.