A great quote from the ARIN-PPML mailing list (not written by me):... and the author continues, to note that "Yes, it has holes like any metaphor does, but it's good enough to explain to a non-techie the various problems and options available -- and their costs." ... INDEED!
"IP address space is allocated under homesteading rules: you must prove you need it before you can get it. The amount of unclaimed IPv4 space is running out, which means soon we'll need to buy it from someone else, though the homesteading rules will still apply. However, there's a new continent out there called IPv6, which has all the land we need for cheap, but it's on the other side of the ocean so we'd need ships called 'NAT-PT devices' for our customers to get back and forth until everyone else has migrated over too. Or, we can build taller buildings, using NAT construction, here on the IPv4 mainland to house more customers on less land, but those buildings are more expensive to buy and maintain, fall down frequently, and one day we'll need those same NAT-PT ships to get to all the folks over on the IPv6 continent."