The IPv4 unallocated address pool is projected to be exhausted.

Internet Protocol version 4 (IPv4) is the fourth revision in the development of the Internet Protocol (IP) and it is the first version of the protocol to be widely deployed. Together with IPv6, it is at the core of standards-based internetworking methods of the Internet. IPv4 is still by far the most widely deployed Internet Layer protocol, as IPv6 is still in its infancy of deployment.
IPv4 is described in IETF publication RFC 791 (September 1981), replacing an earlier definition (RFC 760, January 1980). The United States Department of Defense also standardized it as MIL-STD-1777.
IPv4 is a connection-less protocol to be used on packet-switched Link Layer networks (e.g., Ethernet). It operates on a best effort delivery model, in that it does not guarantee delivery, nor does it assure proper sequencing, or avoid duplicate delivery. These aspects, including data integrity, are addressed by an upper layer transport protocol (e.g., Transmission Control Protocol). IPv4 provides only header integrity achieved with a checksum.
http://en.wikipedia.org/wiki/IPv4