Dividing the indivisible

The “proper” approach would be to call up the provider and ask for more; there is no reason to limit IPv6 assignments to end-users to just a single /64, and in fact the current best practices document (RFC 6177) recommends “giving home sites significantly more than a single /64”.

But what if changing the ways of your provider does not seem to be a feasible option, but you still want to have at least a semi-comfortable experience (i.e. not being forced to bridge everything into one L2 LAN)?

This simple HOWTO explains how you can have one “primary” network with functional SLAAC, and an arbitrary (16 in my example) number of “secondary” networks, where only DHCPv6 will be available. Full two-way routing between all these networks will work without any issues.

Let's assume we have the following.

What we do is…

Carve out a /108 out of the /64, and split it into up to sixteen /112 networks.

Let's say this network will be 2001:db8:abcd:cdef:ffff:ffff:fff0::/108.

But if hosts on the main LAN assume they have the /64 directly accessible on-link, how will they know they need to reach some subnets that are technically within the /64, via the router?

The answer is: from a more-specific route in RAs!

For the Dibbler DHCPv6 server (the rest of the config file is omitted).

iface "eth1" {
 class {
   pool 2001:db8:abcd:cdef:ffff:ffff:fff1::/112
 }
}

iface "eth2" {
 class {
   pool 2001:db8:abcd:cdef:ffff:ffff:fff2::/112
 }
}

iface "eth3" {
 class {
   pool 2001:db8:abcd:cdef:ffff:ffff:fff3::/112
 }
}

interface eth0 {
    AdvSendAdvert on;
    prefix ::/64 { };
    route 2001:db8:abcd:cdef:ffff:ffff:fff0::/108 { };  # <- the main secret sauce in this whole recipe
};

interface eth1 {
    AdvSendAdvert on;
    AdvManagedFlag on;
    prefix ::/112 { };
};

interface eth2 {
    AdvSendAdvert on;
    AdvManagedFlag on;
    prefix ::/112 { };
};

interface eth3 {
    AdvSendAdvert on;
    AdvManagedFlag on;
    prefix ::/112 { };
};

Simple.

• eth0: 2001:db8:abcd:cdef::1/64
• eth1: 2001:db8:abcd:cdef:ffff:ffff:fff1::1/112
• eth2: 2001:db8:abcd:cdef:ffff:ffff:fff2::1/112
• eth3: 2001:db8:abcd:cdef:ffff:ffff:fff3::1/112

The clients on eth1/2/3 will constantly complain into 'dmesg' about “IPv6 addrconf: prefix with wrong length 112”. This is harmless and can be ignored.

Hosts from your primary LAN assume that the whole /64 is their own personal playground, and that they can pick any address they want. In theory this can lead to address collisions with some hosts on your secondary networks. But in practice chances of this are astronomically low.

1. We ruled out possibility of collision with a EUI-64 address by using ff:ff in the middle of our /108 (those will always have ff:fe);
2. The probability of “Privacy Extensions” address suddenly having ffff:ffff:fff in it, is… well, calculate yourself, seems pretty low too.