As you may have heard, we are running out of IP version 4 (IPv4) addresses, usually just called IP addresses. These are familiar addresses like 220.127.116.11 (the address for www.bellaonline.com at the time of writing.) In order for a device to connect to the internet, it must have an IP address. If you've been in the IT field for a long time, you've probably heard about this impending crisis before, however while in the past technology like CIDR and Network Address Translation (NAT) have put off the depletion date or people were just crying wolf, at this point (early December 2010), it looks like we will run out of addresses in early 2011. The long term solution is to switch to IPv6, but that is the subject of another article. Even if you are familiar with the switch over to IPv6, do you know what will happen when we run out?
Unused IP addresses are held in a global pool by the Internet Assigned Numbers Authority (IANA). They pass them out to 5 Regional Internet Registries (RIR's) as they need them in blocks of either 1 or 2 /8's (a contiguous block of 16,777,216 addresses defined using CIDR notation.) There are currently only 7 /8 blocks left in the IANA pool. When there are only 5 /8 blocks left, one block will be assigned to each RIR and the IANA will be out of addresses. The American Registry for Internet Numbers (ARIN) is the RIR for North America. ARIN expects that their free pool will deplete in as little as one day (the top 10 internet service providers, for example, could deplete a /8 in one day with legitimate requests) to a maximum of six months. Generally, a /8 lasts ARIN for a few weeks. ARIN is planning to set aside a /10 from this last /8 for use in transitioning to IPv6, which it will pass out in blocks of /28 to /24, allowing organizations to make services like DNS and e-mail available on both stacks. (Other RIR's likely have similar transition plans in place.) However, once the remaining addresses are gone, like it or not, all future network devices will either need to use IPv6 or use private addresses behind possibly multiple layers of NAT.
It is expected that there will be a number of services to provide bridges between content on IPv4 and IPv6, but to some extent that depends on how ready content providers and ISP's are. Eventually, the best thing to do will be for everyone to convert to IPv6, however that is likely to take place over many years.
Note: On November 10, 2010 I had the privilege of hearing Richard Jimmerson of ARIN speak at the LISA conference, much of the information in this article comes from that talk and the ensuing discussions.