> Even though AWS may own 3/8, you probably won’t see anything in the 3.15/16 space advertised for the next 60 years.
Why is that? Did i miss something?
That's the long lived edge case – I didn't really go into detail, but what the heck.
AIUI, assigning a new IP address to a mainframe isn't really hard: I'm told by reliable sources it's "just JCL, like everything else".
But assigning an IP address to a machine that's had the same one since the mid-eighties? When it has machines talking to it that don't have the concept of "DNS resolution", running programs whose source code was lost a decade ago? And all those things are the revenue critical systems? That's hard.
Talking with colleagues that have done large IP range sales before, this is pretty much the dominant strategy – your agreement sells all your contiguous space, then you have an immediate lease back provision for carved out space that gives you ~decade(s) of lease time to unwind whatever business processes exist that require those legacy IPs remain around.
That subnet had a handful of mainframes that had the same IP address for decades. When we migrated a handful of systems off those mainframes and onto what were then called "minicomputers" (heh!) they ended up in the same subnet.
Those mainframes ran the ERP system for an entire business unit. Every accounting application lived in or talked to a database in that space. Manufacturing information systems that ran assembly lines called into it. The IVR systems talked to systems in that space. Hell, public facing systems called into it (behind a thick layer of indirection :))
Basically, the systems behind that IP space were responsible for ~5 billion in annual revenue, and it was significantly cheaper to lease back those IPs from AWS, do absolutely nothing with them (in AWS), and advertise routes internally for the systems that still remain on those old IPs.