The only reason I can think of is psychological: People don’t want to learn new things, so they find reasons to dislike the new thing to be able to pretend they don’t need to learn it.
Its been decades, but still IPv6 is deployed (at best) as a parallel network that effectively doubles management/maintenance overhead. And at worst sites just add an IPv6 reverse proxy.
I should be able to have a router with a ipv4 network and multiple ipv6 networks without losing access to the ip4 network. The ip6 network should be reachable via port natting on the ip4 IP
I.E I have three networks on my router, two on ipv6
1000:2000:3000:4001::1/64
1000:2000:3000:4002::1/64
192.168.0.1/24
the ipv6 only server on
1000:2000:3000:4001::10
should cope with sending a packet to
1000:2000:3000:4002::10
By routing it via the router
It should also transparently convert a target of 192.168.0.3 to ::ffff:c0a8:3 and send via the gateay.
The router should then convert it to 192.168.0.3, with a source of 192.168.0.1, and maintaining a NAT state so return traffic goes back to 1000:2000:3000:4001::10
If there's a service that 1000:2000:3000:4001::10 needs to expose, the router can do a dst-nat on say 192.168.0.1 port 80 and forward to 1000:2000:3000:4001::10 port 80
That way you can comfortably deploy ipv6 wherever possible and not have to worry about ipv4 other than at the router, where you just have the ipv4 subnet
Likewise my ipv6 network can reach 209.216.230.240 by running ipv6 across my network until it comes to a device with an ipv4 address, where it gets natted. Just like it runs across my private rfc19xx ipv4 range before srcnatting at the edge of my network.
IPv6 works best when all addresses are globally routable (whether firewalled off or not). We're all so used to RFC1918 that we forget it was an ugly kludge that fundamentally broke how the internet was meant to work. IPv6 is the fix for that breakage: the address of every individual device can actually mean the same thing everywhere on the internet, as it was meant to be. L3 routing can be stateless again.
The primary intent of IPv6 is to replace IPv4, not coexist with it. Coexistence is transitory and not worth optimizing for over the future of the internet when V4 is dead.
As for nat being a cludge, lets assume I have a simple small office network with two independent ISPs. Normally I want to send half my users out of ISP1 and half out of ISP2.
If ISP2 fails, I want to send them all out of ISP1, OK there's less bandwidth to go round, but better than having no bandwidth for half my users.
How do I do that with ipv6 without natting (assuming I'm not large enough to be running my own AS and peering with two different providers)
I don't have easy access to multiple V6-PD enabled providers to test this theory, and as someone with quite the neck beard I really don't know how I feel about ceeding this level of control to endpoints. But also, I'm not sure I hate it either.
Oh and don't forget link-local and a ula prefix for your local addressing requirements for pinters and whatnot that shouldn't be using dynamic discovery.
That basically doesn't work with real clients. They'll do dumb stuff like use address from provider A to send through the router advertising addresses from provider B. And take forever to do anything in response to prefixes that are advertised as no longer usable or simply no longer advertised.
V6 devices are expected to understand that and do the right thing, but Windows (10) doesn't, Linux was worse, and I don't remember what Android did and I didn't get around to testing FreeBSD, and that's all the OSes I have.
If you've got experience otherwise, I'd love to know, one of these days I need to setup IPv6 again, but what I'd really like to do is too much work, so I'm IPv4 only for the foreseeable future.
The notion that every endpoint has a globally unique address, with prefixes coming down from some upstream provider is just fundamentally incompatible with how IPv4 networks are designed today.
On a much smaller scale it's analogous to migrating from mariadb to postgres, but the migration takes 50 years and you end up realistically maintaining both.
There's a whole generation of people and experience that wasn't able to enter the discussions in 1993 and is available now. Maybe new eyes or experienced eyes would have a better solution.
The problem is clearer now than then. IPv6 makes many changes to address many problems, but maybe only address space is a real problem, because only address space seems to be motivating people to join IPv6.
Of course, the downside is not a lot of people want to be on three parallel networks, it's a lot of work. But perhaps, if that third network were more desirable and easier to implement, it could get larger coverage in a shorter amount of time than IPv6. Sort of like how TLS 1.1 never had more users than TLS 1.0, but TLS 1.2 overtook them both.
You would need real consensus and commitment from lastmile ISPs, backbone ISPs, networking vendors, OS vendors, CPE vendors, mobile networks, content providers, etc, though and that's tough. The consensus on IPv6 seems to be clearly, we'll do it eventually, when we really have to (which for some networks was 2012, and some networks is now, and some networks seems to be never)