IPv6 is here. Finally it seems it has gotten some momentum. It will be some time before we can actually discard of our IPv4 addresses, I guess. In the mean time you need to start handling IPv6 traffic.
If all your web traffic goes through Varnish, you're in luck. Varnish has had IPv6 support since 1.0. So, all you have to do is just do IPv6 support on the those and nobody will know that the inside of your network is all dusty and ugly and talking IPv4 only. Nice, eh?
If your server has an IPv6 address and you start up Varnish it will automatically listen to any IPv6 interfaces there is. If you want to specify what addresses it should listen to the syntax is simple:
varnishd -a [2001:67c:2804:1001::c21f:279b]:80
Note the square brackets around the address. The reason for this somewhat odd syntax is due to the somewhat odd decision to use colons in the address. You can read Poul rant here.
What are you waiting for? Get crackin!