It turns out that the house router gets the range of IP addresses with the prefix /64 which seems to be not divisible into multiple subnets.
An IPv6 address is eight groupings of numbers:
- Network address - the first three groupings of numbers (first 48 bits) in the subnet mask
- Subnet address - the fourth grouping of numbers (the 49th through 64th bits) in the subnet mask
- Device address - the last four groupings of numbers (the last 64 bits) in the subnet mask
Source: IBM Knowledge Center – Subnet masks (IPv4) and prefixes (IPv6)
The ISP seems to be not following the Recommendations on IPv6 Address Allocations to SitesRecommendations on IPv6 Address Allocations to Sites.
In particular, it recommends the assignment of /48 in the general case, /64 when it is known that one and only one subnet is needed and /128 when it is absolutely known that one and only one device is connecting.
Further reading
Alternative
I got IPv6 working by setting up the router as a switch. However, this means that all the devices and traffic on the subordinate (flat) network are exposed to the upstream (house) network.
Instructions
- Manually set the LAN IPv4 address of the subordinate router. You will need it to access the configuration of that router, at least the parts that will be applicable after the transition, for example Wi-fi.
- If you can influence the upstream router configuration, reserve a static IPv4 address in the configuration of the upstream router.
- Otherwise set an address from a different range, for example if the upstream router supplies 10.0.x.x addresses, use 192.168.x.x. You’ll have to configure your computer to use the same range whenever you need to access the router configuration.
- Turn off the DHCP server (both IPv4 and IPv6) of the subordinate router.
- Disconnect the upstream cable from the WAN port and connect it to a LAN port.
Based on: http://it.cestuji.info/modem-jako-switch.php (in Czech)
