It's an internal network if it's inside your networking perimeter. The type of addresses you use on it are irrelevant to that.

Most enterprise networks need to be routed to other networks -- either other networks inside the enterprise, networks inside other enterprises or the internet. To make that work sanely all of the networks involved need to use address ranges that don't overlap. RFC1918 is pretty much the definition of overlap, and using it makes things way more complicated than they would otherwise be.

It's fine for IPv6 engineers to want to make everything directly routable, but in IPv4 the difference still matters, and therefore that's the scenario we need to compare against.

The IPv6 enterprise argument is 'You might one day have to renumber some devices because some other enterprise might have set up overlapping addresses, therefore you definitely have to renumber everything now, and oh, every time the ISP decides they don't like you, because private addresses are icky'. Obviously that's a hard sell.

I’m confused why would this happen?

> The IPv6 enterprise argument is 'You might one day have to renumber some devices because some other enterprise might have set up overlapping addresses, therefore you definitely have to renumber everything now, and oh, every time the ISP decides they don't like you, because private addresses are icky'. Obviously that's a hard sell.

If you’re an enterprise you would just get your own IPv6 block assignment. It’s not like they’re expensive or hard to get hold of.

Companies already buy and manage domains, this is no different. Using a domain you don’t own is just plain stupid, same applies to IPv6 ranges.

> It’s not like they’re expensive or hard to get hold of.

For context, apparently a block of 79,228,162,514,264,337,593,543,950,336 addresses (/48) is about 100$ a year.

79 octillion addresses ought to be enough for anybody.

Or you could just say screw it, and use RFC4193, and then all you need to ever worry about is prefix remapping at a few key touch points. Particularly if you don't care about external routing in the first place.

16 million is way too few because of collisions and the birthday paradox. Even with a tiny corporate network, you are likely to connect to other companies’ networks, and, with private addresses, no one coordinates allocations. Collisions are quite likely.

Also, the major players probably have more than 16 million devices, especially if you could VMs.

Every sane large internal network is segmented. I doubt all the "more than 16 million devices" need to talk to every other device. The IPv4 limit is an annoyance, but not a big one for the typical internal networks.

When my internal network connects, via any non-NATed means, to a vendor’s internal network, there is a chance that my addresses conflict with theirs. If the vendors have even a few tens of clients, this risk is quite high.

This is especially true because people tend to use the same private addresses over and over. How many devices are 192.168.1.1 vs. 192.168.52.123 ?