Back in the Globalcenter (colo/hosting) days here at Global Crossing we used to say that there were three phases of the space being sold.  The first phase was where customers could buy a rack and reserve several additional adjacent racks for future use.  Once the facility was completely sold out (as little as two months after opening) we would go back to the customers that reserved space and offer them first right of refusal.  They had to pay for the racks even if they couldn't occupy them.  Once all racks were actually being paid for, there was yet another phase...the one where you go back and plead with customers to give up space they aren't using even though they are paying for it.  Once complete, the facility was officially 100% sold out and filled to the gills with servers.

I was reminded of this when I was thinking about IPv4 exhaustion predictions and what they really mean.  There are a couple of predictions out there...one of them is by the guys over at Command Information right on their home page.  As of the time of this writing, it has 748 days left on the counter.  I found another one that I think is probably a little more accurate here that gives us more like 1200 days. Pretty neat, huh?  So what are these address exhaustion predictions based on, and what do they really mean to us?

Just like selling datacenter space involves several levels of sold or "gone", and by my estimation, it will have about 4 levels.

The first level is when IANA, the source of all addresses, runs out of space.  IANA doesn't give space directly to end users it gives space to regional registries such as ARIN (North America) and RIPE (Europe), so even after they run out, everyday companies and ISP's will still be able to request space if their local registry has space.  ETtE (Estimated Time to Exhaustion) at this level is 2-3 years. 

Eventually, the regional registries will have no more addresses to give.  ETtE is 3-4 years.

As a buyer of Internet services, however, you often don't need to go directly to a registry unless your need is large, and of course no one is going to be making any large requests in such times as they'll be making a small request to work in conjunction with a liberal use of NAT (Network Address translation).  As such, you will still be able to get an allocation from your ISP.  This could be a very interesting time, as ISP's that have space to allocate may have a competitive advantage over those that do not and may be able to charge a premium as a result!  My ETtE on this one is 4-6 years.

The last and final phase, much like the selling of datacenter space, will involve money and perhaps some begging.  The black market, perhaps Ebay, will become the only place that one can acquire addresses.  They may not even be transferable across upstream providers, but that won't keep people from trying to trade them.  Eventually the cost of IPv4 address space will be more than someone is willing to pay for and the v4 Internet will officially be sold out.  My ETtE for this final stage is up to 10 years from now.

I imagine that by this point in time there will probably be some way for a user to "borrow" someone elses NAT to convert their v6 addresses to v4 ones, either a provider hosted NAT or some that sit in the public domain.  If such a thing does not exist, then the Internet will definitely be a smaller place for those with v6-only space.  Seeing as nature abhors a vacuum, I doubt this will be the case.