When a site is reachable using both IPv4 and IPv6, Linux by default has a preference which is roughly:

1. Native IPv6
2. Native IPv4
3. 6to4 tunnels

To change this preference, edit ‘/etc/gai.conf’
uncomment most of the labels:

label ::1/128       0
label ::/0          1
#label 2002::/16     2
label ::ffff:0:0/96 2
label fec0::/10     3
label fc00::/7      4
label 2001:0::/32   5
label ::/96         6

(note that 2002::/16 [6to4] is left commented out)

and have the precendence configured as:

precedence  ::1/128       50
precedence  ::/0          40
precedence  2002::/16     30
precedence ::/96          20
precedence ::ffff:0:0/96  10
#
#    For sites which prefer IPv4 connections change the last line to
#
#precedence ::ffff:0:0/96  100

For these changes to have effect, restart the appropriate applications eg. Firefox.

6to4 is a tunneling protocol for using IPv6 over an IPv4 connection, and it’s configuration on Linux is well described.

Note: this can only apply to interfaces which have a public IPv4 address

A 6to4 tunnel can be configured using the following shell script

#/bin/bash

# set the interface name
if=wlan0

# calculate the IPv6 address
ipv4=`/sbin/ifconfig $if | grep "inet addr" | sed -e 's/^. *inet addr://' | sed 
-e 's/ .*$//'`
ipv4s=`echo $ipv4 | tr "." " "`

ipv6=`printf "2002:%02x%02x:%02x%02x::1" $ipv4s `

case "$1" in
  start)
/sbin/ip tunnel add tun6to4 mode sit ttl 128 remote any local $ipv4
/sbin/ip link set dev tun6to4 up
/sbin/ip -6 addr add $ipv6/16 dev tun6to4
/sbin/ip -6 route add 2000::/3 via ::192.88.99.1 dev tun6to4 metric 1
# configure firewall
/sbin/ip6tables -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT
/sbin/ip6tables -A INPUT -j DROP
    ;;
  stop)
/sbin/ip -6 route flush dev tun6to4
/sbin/ip link set dev tun6to4 down
/sbin/ip tunnel del tun6to4
# clear firewall
/sbin/ip6tables -F INPUT
  ;;
  *)
    echo "usage: ipv6 {start|stop}"
    exit 1
esac

exit 0

This will create a new interface ‘tun6to4’ which will be used for IPv6.

See here for link preference.