[Podcast] The back of the class: Looking at 240/4 reachability

In his regular monthly spot on PING, APNIC's Chief Scientist Geoff Huston discusses a large pool of IPv4 addresses left in the IANA registry from the classful allocation days in the mid-1980s. This block, from to , encompasses 268 million hosts, a significant chunk of address space equivalent to 16 class-A blocks, each of 16 million hosts. It seems a shame to waste it. How about we get this back into use?

Back in 2007, Geoff, Paul Wilson, and I submitted an IETF draft proposing the removal of these addresses from their 'reserved' status in IANA, so they could be used to supplement the RFC 1918 private-use block. At the time, we believed this was the best use of the addresses due to their apparent non-routability on the global Internet. Most IP network stacks at that time shared a lineage with the BSD network code, which was developed at the University of California and released in 1983 as BSD 4.2. Subsequent versions of this codebase included a two- or three-line rule inside the kernel that checked the top four bits of the 32-bit address field and refused to forward packets with these bits set. This rule reflected the IANA designation marking the range as reserved. Ultimately, the draft did not achieve consensus.

A more recent proposal by Seth Schoen, David Täht, and John Gilmore, introduced in 2021 and still under development, suggests using the address in global unicast space rather than assigning it to the RFC 1918 internal non-routable range. The authors believe that the critical filtering mechanism in devices has been lifted and is no longer prevalent in BSD and Linux-derived codebases. This approach mirrors how the address space has already been used within data centres.

Geoff has been measuring the reachability of this address space using the APNIC Labs measurement system and a prefix from the range, which was temporarily assigned and routed in the Border Gateway Protocol (BGP). The results were not encouraging, and Geoff believes that the routability of this range continues to be a significant challenge.

Read more about 240/4 on the APNIC Blog, and the IETF Datatracker website:

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