ICANN’s President and Chief Executive Officer, Rod Beckstrom said “This is a major turning point in the on-going development of the Internet. No one was caught off guard by this. The Internet technical community has been planning for IPv4 depletion for some time. But it means the adoption of IPv6 is now of paramount importance, since it will allow the Internet to continue its amazing growth and foster the global innovation we’ve all come to expect.”

IPv4 uses 32 bits for the addressing. This makes 4.3 billion addresses. Only 3.7 billion IPv4 addresses are usable by ordinary Internet access devices. The others are used for special protocols, like IP Multicasting. Today, none of those 3.7 billion IPv4 addresses remain unallocated.

IPv6 has a 128-bit address space, which is 340 undecillion addresses (340,282,366,920,938,463,463,374,607,431,768,211,456).

You can find more on IPv6 addresses here. You can also find  information about the RFCs in the TCP/IP Guide.