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If you have a Kimsufi Dedicated, you got a full /64 not just /128 which is a single IP, lets use it.

Next, we need to add IPv6 to our vmbr2, which the VM should be connected to.

iface vmbr2 inet6 static
     address 2601:xxxx:xx:xxx::2
     netmask 64
     bridge_ports none
     bridge_stp off
     bridge_fd 0
     post-up echo 1 > /proc/sys/net/ipv6/conf/all/proxy_ndp
     post-up echo 1 > /proc/sys/net/ipv6/conf/all/forwarding
     post-up echo 1 > /proc/sys/net/ipv6/conf/default/forwarding
     post-up /sbin/ip -f inet6 neigh add proxy 2601:xxxx:xx:xxx::2 dev vmbr2
     post-up /sbin/ip -f inet6 neigh add proxy 2601:xxxx:xx:xxx::3 dev vmbr0
     post-up /sbin/ip -f inet6 route add 2601:xxxx:xx:xxx::3 dev vmbr2

The next thing you need to do, restart your network on your Dedi with:

 service networking restart

Now Login into your VM and add this to your /etc/network/interfaces:

iface eth0 inet6 static
     address 2601:xxxx:xx:xxx::3
     netmask 64
     gateway 2601:xxxx:xx:xxx::2

Restart your Networking on the VM:

 service networking restart

If it dosen't respond after a while, try to Reboot your VM over Proxmox. Now we still need to add IPv6 DNS Server to our /etc/resolv.conf:

2001:4860:4860::8888
2001:4860:4860::8844

These are the IPv6 Nameservers from Google but you can use different ones.

If you run:

ping6 google.com

You should get a response and you should be able to reach your VM under this IPv6 which you configured earlier.

Sources used: http://hw7.net/?p=252