In reply to a post by iand:

As others have said, the ipv4 address is associated with the router, but the ipv6 address is associated with the device. A device when restarted (PC) by default gets a new ipv6 address. Mostly mine changes each time. This is so your address can not be used by websites to track your usage. If you want to use the same address all the time, then you can config your device to use a single address. This is normally set on ipv6 servers.

That is quite a cool feature of IPV6 to be honest. It makes it sound more secure than IPV4.

I got native ipv6 working on my asus router now, it took some effort.

1 - a new custom firmware had to be made, because by default it only will work with /64 on dhcp-pd.
2 - I then had to create a dhcp6c.conf to use as for for whatever reason one is not been generated automatically.
3 - finally I had to customise radvd.conf as it was trying to use a /56 prefix on my lan which fails, it insists on a /64 prefix.

After those 3 steps it is working tho.