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 back in the mid 1980s. This block, from 240.0.0.0 to 255.255.255.255 encompasses 268 million hosts, which is a significant chunk of address space: it's equivalent to 16 class-A blocks, each of 16 million hosts. Seems a shame to waste it, how about we get this back into use?

Back in 2007 Geoff Paul and myself submitted An IETF Draft which would have removed these addresses from the "reserved" status in IANA and used to supplement the RFC1918 private use block. We felt at the time this was the best use of these addresses because of their apparent un-routability, in the global internet. Almost all IP network stacks at that time shared a lineage with the BSD network code developed at the University of California, and released in 1983 as BSD4.2. Subsequent versions of this codebase included a 2 or 3 line rule inside the Kernel which checked the top 4 bits of the 32 bit address field, and refused to forward packets which had these 4 bits set. This reflected the IANA status marking this range as reserved. The draft did not achieve consensus.

A more recent proposal has emerged from Seth Schoen, David Täht and John Gilmore in 2021 which continues to be worked on, but rather than assigning to RFC1918 internal non-routable puts the address into global unicast use. The authors believe that the critical filter in devices has now been lifted, and no longer persists at large in the BSD and Linux derived codebases. This echoes use of the address space which has been noted inside the Datacentre.

Geoff has been measuring reachability at large to this address space, using the APNIC Labs measurement system and a prefix in 240.0.0.0/4 temporarily assigned and routed in BGP. The results were not encouraging, and Geoff thinks routability of the range remains a very high burden.

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

• Looking for 240/4 addresses (Geoff Huston, APNIC Blog September 2024)
• Re-delegation of 240/4 from "future use" to "private use" (expired IETF draft, 2008)
• Unicast use of the formerly reserved 240/4 (active IETF draft, 2024)