Summary

  • Matthias Wichtlhuber, Remco van Mook and Wolfgang Tremmel coauthored RIPE proposal 2023-01, which changed a new internet exchange’s default peering-LAN IPv4 assignment from /24 to /26.
  • The rule, implemented on 14 September 2023, lets an IXP request up to /24 after using more than half its initial /26, but requires the old assignment back. That upgrade can require a coordinated renumbering of members’ routers.

The cheapest IPv4 address is the one you never hand out. At an internet exchange, that saving can produce a more expensive email thread later. The exchange's peering LAN gives its members the addresses they use to meet each other. If the LAN outgrows its block, the exchange may have to change those addresses on routers it does not own.

Matthias Wichtlhuber described that bill at RIPE 86. He was presenting a proposal with Remco van Mook and Wolfgang Tremmel to shrink the first IPv4 block given to a new IXP. RIPE's old default was /24; their 2023-01 proposal chose /26. That is one quarter as many addresses at the starting line. A small exchange consumes less of the reserved IXP pool. A growing exchange eventually has to move.

The rule's trigger is precise. After more than half of the initial /26 is in use, an IXP may ask for up to /24 and return the old assignment. If it later needs more than /24, it must again return the current block and can receive up to /22 under the policy's utilisation conditions. RIPE-804 confines this reserve to peering-LAN use and requires the IXP to have applied for, or already received, IPv6 space for that LAN.

Wichtlhuber did not pretend that returning a prefix is the same as returning a borrowed chair. The exchange must alter its own configuration, then persuade member networks to change router interfaces under their control. At RIPE 86 he spoke about chasing members for months when renumbering goes badly. That is the policy's real price tag: addresses are saved centrally, while part of the work is paid in distributed engineer hours.

There was a reason to accept the trade. RIPE NCC's contemporaneous impact analysis projected the old IXP pool could empty in the second half of 2029 under a linear scenario; a smaller default could carry it into the 2030s. Those were planning projections, not an inventory of what happened afterward. The proposal was accepted and implemented on 14 September 2023. RIPE NCC's current request guidance still describes the /26 start and the return-and-replace path.

A policy can conserve numbers without making the migration cheap. Wichtlhuber's contribution was to put both sides in the same room: leave more IPv4 for the next exchange, and admit that an exchange which grows may one day spend a season persuading its neighbours to change a subnet mask and an address. The reserve gets more birthdays. Operators get the invitations to the moving party.