Exactly - the engineers who authored RFC 4193 were super familiar with the problems of RFC 1918 space - and so they were super, super clear on exactly how you needed to allocate space from the FD/8 network. The details are very clear, and the RFC is an awesome read - but the net-net is you can generate a /48 - which means you have 2^16 sub-networks to work with (each subnetwork having effectively unlimited number of hosts allowed on it) - and you can connect with 1,000 other companies that have done the same thing, and there is only a 4.54*10^-07 chance of collision.
I know IPv6 network engineers despise RFC 4193 - but, it offers extraordinary flexibility around addressing - particularly if you have large networks that you want full control over. At my last gig, we deployed over 25 million nodes in RFC 4193 space, and it worked like a charm.
- Both corporations probably started allocating from the top
- Both corporations probably allocated huge subnets.