Often things are allocated and remain unused, but cannot be reallocated. So companies end up with large parts of 10/8 reserved. I recently had to start using some weird 100 addresses (some reserved router protocol) for this reason. It doesn't help that a lot of AWS stuff wants to consume an IPv4 for no good reason (IE, the machine gets an IPv4 but isn't reachable from the internet).

100.64.0.0/10 is reserved for CGNAT providers to assign to CPE. If you don't connect to a CGNAT then there's nothing stopping you from using it, however. Tailscale hands out addresses in 100.100 for that reason (deconflict with customer private networks)

https://en.m.wikipedia.org/wiki/IPv4_shared_address_space

Okay, so I may have left out a key detail that these "companies" were large ISP's and had massive networks of their own with hundreds of thousands of devices, let alone customer devices on that same network that needed to be managed.

When every set top box, cable modem, customer wifi gateway, and all of your routers/management interfaces need RFC1918 space it goes real quick, real fast.

It gets even worse when you are have to have a lab where you can setup infrastructure that looks/feels exactly like production and thus needs similar setups, now do that for testing/staging... and there's doubling of your required IP space already.

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.

>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."

Imperfect foresight and poor subnetting.

Even with the ability to renumber RFC1918 is just not that large, and when you need to summarize routes/BGP you can't easily re-use empty space from one location in another because it'll blow up your routing tables.

> But how in earth does one run out of 10.0.0.0/8 IPs, outside of a company merger?!

Twenty company acquisitions.