I was looking for configuring Slackware to use ipv6 but all instruction I found speak about using an ipv6 tunnel that encapsulate ipv6 request into ipv4 packet and send them to an external router that extracts ipv6 request and sends a reply (or, at least, this is what I understood).

Is that necessary? Isn't there a way to configure a pure ipv6 system? If yes, could you please point me to a guide that clearly explain how to enable ipv6 without this trick?

I would like to configure my Slackware desktop at first, and then do the same with my Centos server.

EDIT: maybe I gave you too few information. Sorry. I'll write some more information thanks to the posted guide.

~$ test -f /proc/net/if_inet6 && echo "Running kernel is IPv6 ready"
Running kernel is IPv6 ready

So, it seems ipv6 is enabled in my kernel.

Some other output from ifconfig, route and /etc/resolv.conf content (with opendns):

~$ /sbin/ifconfig wlan0 | grep inet6
inet6 addr: fe80::21f:3bff:fe60:cc5b/64 Scope:Link

~$ /sbin/route -A inet6 | grep wlan0
fe80::/64                      ::                         U    256 0     0 wlan0
ff00::/8                       ::                         U    256 0     0 wlan0

~$ cat /etc/resolv.conf
inet6
nameserver 2620:0:ccc::2
nameserver 208.67.222.222
nameserver 208.67.220.220

But still, with ping6 I can only ping localhost (::1). Everything else is unreachable. Normal ping works fine.

That is why I was asking if I am obliged to use a tunnel.