After the adoption of the Global Policy for Post Exhaustion of IPv4 resources by the ICANN board, the ARIN board has decided to return to IANA some of the resources that it had received back from legacy address address holders.  All of this space is the equivalent of a /8.  The largest part of this block came from Interop who returned almost a /8 to ARIN in 2010.  The long-term effect of this will bring ARIN’s IPv4 exhaustion date closer to RIPE’s exhaustion date which is expected this year.

From: Geoff Huston’s IPv4 Address Report

Presumably IANA will now proceed to reallocate this address space to all 5 RIRs according to the global policy.  However, given that some RIRs currently have a sufficient free pool one would expect them to decline the reallocation of space, thus a majority of the space seems likely to go to either APNIC or RIPE.  APNIC’s current IPv4 policy, however, would only allow blocks of size /22 or smaller to be allocated.

From a practical perspective, it seems like it would have been more reasonable for ARIN to just return the largest blocks and reallocate the small blocks to new or existing IPv4 address holders.  However, the final /8 allocations process by chance ended up giving ARIN a much larger supply than was needed compared with the other RIRs.  Thus, it seems likely the ARIN board felt compelled to “right this wrong” as well as to continue its previously existing operational procedure to return to IANA IPv4 space received from organizations prior to ARIN’s formation.