AS14371 PBNET

The reverse names encode an address in letters instead of digits, and the letter advances one step for every step of the third octet.

What it's used for

• A rural cooperative's broadband network. 36,096 addresses across ten IPv4 prefixes plus one IPv6 prefix, 2600:6400::/28, with 22 observed BGP neighbours.

Notable

• Eight blocks carry netnames running PBTL-100 through PBTL-107, registered to Paul Bunyan Communications, the older ones alongside Paul Bunyan Telephone.
• The remaining two belong to other companies: AFTERNORTH to AfterNorth and NET-BRECK to BRECKENRIDGE SKI CORPORATION.
• Thirteen of eighteen sampled addresses returned a name, nearly all of the form jc0.ips.PaulBunyan.net, jd255.ips.PaulBunyan.net, jf254.ips.PaulBunyan.net.
• Two addresses answer under a different domain entirely: 65-18-205-0.a51.co.

Interesting

• Compare jc0 at 66.171.42.0 with jd255 at 66.171.43.255 and jf254 at 66.171.45.254. The trailing digits are the final octet and the letter walks forward as the third octet does, so the whole address is recoverable from a label of four characters. It is the address written in a shorter alphabet.
• The same trick runs across every block with its own starting pair, which is why one zone contains ey254, fh255, gaf255 and ic254 without collision.
• The domain is spelled two ways in the same zone, capitalised in most labels and lowercase in one. DNS does not care, but anything doing a string comparison downstream will.
• Sequential netnames from PBTL-100 upward are a cooperative that numbered its allocations as they arrived and never renumbered. The ski corporation's /24 is what a small ISP looks like when it routes a neighbour's block as a favour.