Under all circumstances: I'm sure we've all been faced at some stage with the unplanned need to route between two RFC1918 subnets, only to find they collide, in situations as simple as a home VPN connection dialling into the office LAN, up to corporate mergers involving hundreds of thousands of desktops.

A central address registry along with 'public' allocations is the only way to avoid this kind of mess. The fact that public addresses are currently scarce doesn't make having unified addressing any less desirable (just presently impractical).

Sorry, I was imprecise. I meant, under what circumstances today is it the best thing to spend routable IP addresses for machines that aren't exposed directly to the Internet? I'm getting directly at the practicality of these schemes. Obviously, if routable IP addresses were easy to get, there would be lots of cases where it would make sense to use them.

I just gave you a reason. Without coordination of private network addressing, those networks essentially speak different protocols, needing horrendous transforms like NAT which only works in specific situations and myriad crap over the application layer (like DNS views) in order to get them to talk.

If anything, today, networks are more likely to end up interconnected than they were in 1994.

> under what circumstances today is it the best thing to spend routable IP addresses for machines that aren't exposed directly to the Internet

Under the circumstance that you have a spare /8 you aren't using for anything.

You could argue that "the best thing" in that case would be giving, selling, or leasing pieces of the /8 to someone who needs the space. But maybe you aren't in the philanthropy business, so giving's out.

And maybe the prices aren't high enough for your taste, so selling or leasing are out. Or maybe there's simply not enough demand on the market to absorb 16m addresses. Or maybe the terms of the allocation agreement say you're not allowed to transfer them, and you're afraid that trying to do so will give the Internet people a justification to give them to someone else.

> if routable IP addresses were easy to get, there would be lots of cases where it would make sense to use them

If your organization has a spare /8, routable IP addresses are easy to get for you, so these cases do make sense, for you.