IPv6 Sub Nets

[Doug Hardie]

-- Doug

> On 26 June 2021, at 19:02, Louis Kowolowski <louisk at cryptomonkeys.org> wrote:
> 
> On Jun 26, 2021, at 8:25 PM, Doug Hardie <bc979 at lafn.org <mailto:bc979 at lafn.org>> wrote:
>> 
>> I am trying to setup an IPv6 environment.  There is a primary router (A) that receives a /48 prefix via DHCP6 from the ISP.  That router configures itself properly via dhcp6c.  It also creates 2 LAN /64 prefixes and creates EUI-64 addresses on the two LAN interfaces.  One of those interfaces is connected to a second router (B), among other devices.  The B router receives the prefix via SLAAC and creates its own EUI-64 address.  However, that router needs to create a smaller subnet, /72, and distribute it to the devices on that LAN.  I have not been able to figure out how to make that happen.  
>> 
>> Clearly, manual configuration would work, but the prefix received from the ISP can change which would raise havoc with the network.  I suspect that dhcp6s needto be run alongside dhcp6c on router B and then the other devices run dhcp6c.  However, I don't see how to get the prefix that dhcp6c receives on router B to the dhcp6s process on router B.  I believe I am missing something, but haven't been able to find it.  Thanks,
>> 
> Speaking generally
> 
> First: I believe you’re looking to do DHCP prefix delegation (dhcp-pd) where you designate a prefix to be used further down the line.

That is what is being used between the ISP and router A.  However, router Bs dhcp6s needs to find that prefix.  About all I have come up with is some sort of script which monitors the network addresses and then updates dhcp6s.conf and restarts dhcp6s.  Not very elegant though.

> Second: There are valid use cases for prefixes smaller than /64, but they are not best practice, and you will likely find software that doesn’t work for them (examples I’ve found include PDUs hardcoding /64, dhcp not working on prefixes smaller than /64. examples of things I’ve seen that do work: point-to-point links on network equipment using /126 or /127).
> 
> --
> Louis Kowolowski                                louisk at cryptomonkeys.org <mailto:louisk at cryptomonkeys.org>
> Cryptomonkeys:                                   http://www.cryptomonkeys.com/ <http://www.cryptomonkeys.com/>
> 
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