Summary

  • In a 2015 ARIN post, Cathy Aronson described a question from a sizable ISP: should it use 22.0.0.0/8 or 30.0.0.0/8 as extra private IPv4 space?
  • IANA's registry already assigns both /8 blocks to Defense Information Systems Agency. Aronson traced the practical cost of pretending another holder's addresses were empty: blocked reachability in both directions and an urgent renumbering job if the holder routed the space.

The spare room had an owner

The colleague's email sounded like a multiple-choice quiz. According to Aronson's firsthand account, an executive at a sizable ISP wanted to know which of two enormous IPv4 blocks would be safer to use as additional RFC1918 space: 22.0.0.0/8 or 30.0.0.0/8. Aronson wrote back to check that she had understood. Yes, came the answer. Her surprise launched the article; the ISP's name stayed out of it.

The IANA IPv4 registry makes the oddity visible. It designates both 022/8 and 030/8 to the Defense Information Systems Agency as legacy space. The same registry marks 010/8 for private use. The two proposed substitutes were therefore not an overlooked annex to the private range. They were rooms with names on the door, however quiet their corridors might have looked to the operator.

Aronson followed the packet instead of the euphemism. If the legitimate holder began routing one of those blocks, customers inside a network using that same block internally could lose access to the holder's real destinations. Traffic intended for a customer on the squatting ISP could also be trapped or misdirected from the holder's side. The operator's shortcut would become somebody else's reachability problem. Then the operator would need to renumber internal devices quickly, a job that gets less charming with every device added.

Her post records discussions with other operators who were considering or using address space registered to others. It does not identify the ISP that sent the original question or prove that it had deployed either /8. The mechanism does not need that missing name. An address plan made inside one company can collide with the wider network because Internet numbers are shared identifiers. The ledger and the running network eventually meet, usually at the least convenient moment.

The seductive word was “internal.” It made the choice sound private even though the address was globally unique to another registrant. Aronson's point was operational, not ceremonial: a borrowed number can work beautifully right up to the day its owner uses it where you can see it. At that point the network team gets a very expensive invitation to change every label it attached to the borrowed space.