The Number Resource Organization warns that less than 10 percent of the IPv4 address space remains; it's time to start adopting IPv6. The warning comes after APNIC, the registry that hands out IP addresses in the Asia-Pacific region, obtained an additional 33.5 million IPv4 addresses from IANA, the Internet Assigned Numbers Authority. This brings the size of IANA's global pool of free IPv4 address space down from 26 to 24 blocks of 16,777,216 addresses ("/8s").

The Number Resource Organization (NRO), which is where the five Regional Internet Registries (RIRs) get together to discuss address-related issues, is taking this opportunity to warn us all that the amount of IPv4 address space that is still free is now below ten percent, so we should all adopt IPv6 post-haste. But only weeks ago, we wrote that a mere 81 percent of the IPv4 addresses is in use, which leaves 19 percent free. What gives?

There are two differences between the number in the earlier story and the one mentioned by the NRO: the NRO talks about /8 blocks rather than individual addresses, and the NRO lumps the unusable address space in with the used address space. Of the 256 /8 blocks that the 32-bit IPv4 address space allows for, only 221 are available for regular use. The remaining 24 unused blocks are 10.9 percent of those 221 usable /8s. And a significant number of the addresses in the /8s given to the RIRs haven't yet been passed on to service providers and end-users. For instance, it will probably take months before addresses out of blocks 1.0.0.0/8 and 27.0.0.0/8 that were given to APNIC show up in actual use—if only to allow firewall administrators to remove those blocks from their "bogon" lists.

There's no doubt about it: APNIC is burning through its address space very fast. According to the policy used in the past, it should have gotten seven /8s to have enough to continue working the next 18 months. With only two new blocks, APNIC will probably have to come back for more after the summer. ARIN, the RIR serving North America, is also running low on address space, and should be getting some new blocks earlier than that.

So we're still running out of IPv4 addresses, and IPv6 is the only medium- and long-term answer. However, it's now too late for a smooth IPv4-to-IPv6 transition, so prepare for a bumpy one where we run out of IPv4 addresses before there is widespread IPv6 adoption.