>10/8 isn't actually that much space. It's only 256 /16s. Either way, mergers are common and it only takes one VPN to another company also using 10/8 to have a problem.

I've had to address that with partners/service providers. It just requires a double NAT to make it work. Which is a pain, but necessity is the mother of invention after all.

It's not just NAT, you may also need split DNS, and it's a constant drag on everything because you keep needing to map between two sets of addresses. And it gets way harder when you have multiple or large companies involved, because you do start running out of RFC1918 space.

It's particularly silly because none of that is necessary at all. v6 means that each side has unique prefixes and you don't need to do any of the above. Even if you need to connect to someone without v6, all you need to do is NAT each partner's RFC1918 space into 64:ff9b:1:N::/96, with a different N for each partner. Then they're all accessible in a single unified address space.

>It's not just NAT, you may also need split DNS, and it's a constant drag on everything because you keep needing to map between two sets of addresses. And it gets way harder when you have multiple or large companies involved, because you do start running out of RFC1918 space.

You won't get any argument from me about the utility of IPv6 over IPv4.

I just pointed out that the scenario presented is workable with IPv4 -- in fact, I've done it more than once.

Sometimes you don't get the choice as to what IP version or addressing scheme you use. Not every environment is a "green field."

Back in the early 'teens, when we contracted/partnered with others that had overlapping IPv4 ranges, if I'd told my boss that "it would be so much easier to do this if we were native IPv6. Given me three-six months to migrate the routers at our dozen locations to dual-stack, replacing any equipment (don't worry, that'll cost less than $250,000) that can't support IPv6, then renumber our dozen class B's, re-implement our 802.1x infrastructure, reorganize our firewall policies and the myriad other tasks we need to do to migrate away from IPv4. By the way, I'll pretty much be engaged in this full-time, so you may need to hire a couple guys to do the work I'd otherwise be doing. And don't forget to tell the C-suite that they can't have the application access they wanted until this is all done."

I suppose I could have said that to my boss. But rather than doing that, then updating my resume as I'd probably have had a lot of extra time on my hands, I said "I'll make it happen. Please give me the contact info for my counterpart at the other organization."