Summary

  • In a 2015 ARIN guest account, Cathy Aronson recalled a colleague at a large ISP asking which of two public /8 blocks, 22/8 or 30/8, would be safer to use as extra internal address space. She described the collision and renumbering risk if the rightful holder used that space.
  • RFC 1918 reserves 10/8, 172.16/12 and 192.168/16 for private internets. Current ARIN records for 22/8 and 30/8 register both blocks to the DoD Network Information Center.

Aronson’s colleague wrote to her with a problem that sounds like an exam question written by the person who forgot to read the chapter. An executive at a sizeable ISP wanted more room for internal devices. Would 22.0.0.0/8 or 30.0.0.0/8 be the safer place to put them? Aronson wrote back to confirm the premise. Yes, came the answer: the plan was to treat one of those blocks as extra RFC 1918 space.

RFC 1918 gives networks three shared private ranges. Neither candidate was on the list. ARIN’s current registration records place both /8s with the DoD Network Information Center. An ISP can make an address look private inside its own routing plan, but it cannot turn another holder’s globally assigned block into common private property. Aronson reported that conversations with operators suggested the temptation extended beyond this one inquiry; her account gives no census of deployments.

The collision is simple enough to draw on a napkin. Suppose internal devices use addresses from a block belonging to someone else. When the holder announces the block publicly, packets from inside the squatting network headed for legitimate destinations in that range can be pulled toward the local impostors. The legitimate users may likewise fail to reach customers behind the squatting ISP. Aronson described the eventual response: renumber a large population of devices, quickly. Every hard-coded reference, DNS entry and operational dependency turns the supposed shortcut into a moving bill.

Her objection was aimed at the incentive, too. IPv4 scarcity makes a large, apparently quiet block look like free floor space. The rightful holder’s later decision to route or transfer it sets the timetable for the squatter’s cleanup. Aronson suggested putting effort into IPv6 instead of building more devices on borrowed numbers. The email did not establish that this ISP actually deployed either /8; the choice it contemplated was revealing enough.