According to Number Resource Organisation [NRO], as of October 2010, [only] less than 5% of the IPv4 address space remains unallocated.
This put a lot of pressure on IPv6 adoption given the increasing rate of new devices connecting to our precious ‘network’. But there’re a lot of benefits coming out of this adoption is spite of the operational cost involved in the migration to a complete, true IPv6 network. Right, the most important one is the large address space : 340 undecillion IPv6 addresses vs ~4.3 billion IPv4 addresses. It means that, currently, each of us – we are around ~6.8 billions in this world – affords only two thirds of an unique IPv4 address, to be mathematically precise. It’s clear that we ran out of the allocation space, even long before the mobile market exploded.
There are a lot of nice comparisons trying to put in real worlds what IPv6 space means, the one I like is coming from Steve Leibson :
So we could assign an IPV6 address to EVERY ATOM ON THE SURFACE OF THE EARTH, and still have enough addresses left to do another 100+ earths. It isn’t remotely likely that we’ll run out of IPV6 addresses at any time in the future
We might hit the limit in case we want to assign an address to all protons but that is not really required for the foreseeable future. Even so, we can still allocate one address to all protons smashed in the CERN collider and the loss is not that significant.
Coming back to higher spheres … let’s see how the IPv6 addresses will be allocated by ISP to an end customer :
A large customer will usually get allocated from his ISP an /48 block, the remaining 16+64 bits being used internally by his intranet devices. Using the default 16 bit subnet part, this means that 65536 (2^16) sub-nets can be configured each of them having 2^64 devices at maximum . Pretty large room!
A cloud having more than 1 quintillion (once again, thanks Walfram) networked droplets (read devices) its a pretty dense one, right?
Last but not least, our life is eased with the new scheme: no more NAT, auto-configured IP based on their ID (former MAC is now enlarged to 64 bit), to name a few.
That’s of course too much for a reasonable user like me and they will actually assign me only a /64 block, leaving me the rest of 64 bits to connect my laptop, tv, mobile and fridge to the public Internet. In case you don’t have enough gadgets to fill up your rightmost 2^64 part, you might be interested in requesting only a dozen of 128 bit addresses – still unique but closer to your needs.
Since we can allocate one IP per atom, I see no reason why we couldn’t do the same for each tiny drop in our clouds.
 Other subnet partitioning is possible, using fewer bits from the subnetid block and cascading them.