maximum ipv4 bgp prefix length of /24 ?

Wed Oct 11 00:20:55 UTC 2023

> On 11 Oct 2023, at 09:43, Delong.com via NANOG <nanog at nanog.org> wrote:
> 
>> As a community, we have failed, because we never acknowledged and addressed the need for backward compatibility between IPv6 and IPv4, and instead counted on magic handwaving about tipping points and transition dates where suddenly there would be "enough" IPv6-connected resources that new networks wouldn't *need* IPv4 address space any more.
> 
> I’m not sure that we never acknowledged it, but we did fail to address it, largely because I think we basically determined that it’s “too hard”.

It’s not actually that hard to do on a small scale, i.e. inside a home CPE with a DNS server and a NAT64 implementation that supports semi static mappings.  It does require IPv4 sites have IPv6 connectivity. You stand up a DNS46 which requests an unused IPv4 address from a prefix block, say 10/8, when there is an IPv6 address without an IPv4 address from the NAT64 with the IPv6 address it needs to be mapped to with an initial NAT64 lifetime value.  The DNS46 would forget the mapping after half that initial lifetime.  The DNS46 would return A records limited half the lifetime or less so they timeout before the NAT64 mapping expires.  The hard part is scaling up to a large client base because not every DNS query results in IP traffic and you need a prefix block big enough to support the add rate of the client base.  Doing this at ISP scale would be interesting to say the least.  This is not theoretical.  It has been implemented in the past though some to the details might differ.

Companies that have gone IPv6-only internally do this with fully static IPv4 to IPv6 mappings and skip the DNS46 step.

So if you have a legacy device that can’t talk IPv6 there is a solution space that allows it to talk to the IPv6 internet.  You need to install it however.  Adding DNS46 to a nameserver is about a days if you already have a DNS64 model.  The hard bit is working out how to talk to the NAT64 implementation.  A good project to put on a Raspberry Pi or similar.

-- 
Mark Andrews, ISC
1 Seymour St., Dundas Valley, NSW 2117, Australia
PHONE: +61 2 9871 4742              INTERNET: marka at isc.org