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My new ISP gives me a public IPv6 address with a /56 prefix to play around:

2001:b:c:d:5:6:7:8/56

I would like to create two subnets divided by a linux server like this:

internet - fritzbox - server - pc

But I need to understand how to propagate a prefix to the internal subnet. So far, I configured the external interface of my server to accept router advertisments and request an IPv6 prefix from the Fritz!Box:

/etc/systemd/network/eth1.network

[Match]
Name=eth1

[Network]
DHCP=ipv6
IPv6AcceptRA=yes
IPv6PrefixDelegation=dhcpv6
IPForward=yes

After a systemctl restart systemd-networkd the externel interface gets an address:

ip -6 addr
  ...
  inet6 2001:b:e:f:1:2:3:4/64 scope global noprefixroute dynamic

Is this correct so far? Also confusing: The prefix of the internet connection and of the interface differ in the second half:

internet: 2001:b:c:d::
server:   2001:b:e:f::

Now the main question: How do I configure the internal interface? Can it request another prefix from the Fritz!Box?

/etc/systemd/network/eth0.network

[Match]
Name=eth0

[Network]
???

My server is running Debian stretch with systemd 239 from stretch-backports (because 232 in stable does not yet support prefix delegation). I removed the ifupdown package because my goal is to learn how to solve this with systemd-networkd only.

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