The fact that we're running out of IPv4 addresses is reasonably well-known by now, but it's not the only declining network resource on the Internet. "AS," or Autonomous System, numbers, which uniquely identify networks for the Internet's BGP routing infrastructure, are also running out, and on a similar timeline. AS numbers, like IP addresses, are allocated by RIRs (Regional Internet Registries), and the last one, based on current trends, will be gone in early 2011.

As this release from APNIC (the Asia Pacific Network Information Centre, the RIR for the Far East) explains, the old AS numbers were 16 bits. A new standard exists for 32-bit AS numbers and, as of Jan. 1, 2009, these 32-bit numbers will be the ones allocated by default, unless a 16-bit number is specifically requested.

But routers and network management software on networks using the new numbers need to be updated. Surprisingly, and impressively, routers on older, 16-bit AS-numbered routes can still communicate with the new 32-bit AS-numbered networks, and vice versa. Click here for more on how they accomplished this neat bit of backward compatibility.