IPv6 Sub Nets

Louis Kowolowski louisk at cryptomonkeys.org
Sun Jun 27 02:02:43 UTC 2021

On Jun 26, 2021, at 8:25 PM, Doug Hardie <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.
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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