The IPv4 protocol used on the Internet today was first deployed on ARPANET in 1983. It uses 32 bit addresses, which limits the number of IP addresses to 4,294,967,296. While this may seem like a lot, that number is being rapidly depleted, even with the boost that NAT provided us.
The replacement, IPv6, improves on IPv4 by switching to 128 bit addressing, which should provide enough IP address space for the foreseeable future. It also makes a number of other improvements including auto-configuration of addresses, simplified processing for routers due to more standardized sizes for packet headers, and additional areas as well.
Even with those improvements, and the impending IPv4 exhaustion, IPv6 has had an extremely slow rollout. The initial design was completed in 1998 but as of the end of 2009 the percentage of users who visited Google with IPv6 connectivity was below 0.25%. Since 2009, adoption has accelerated, with the user saturation increasing from less than 3% to more than...
