The American Registry for Internet Numbers (ARIN) board of trustees has decided to start contacting the CEOs of IPv4 block holders advising them that IPv4 addresses will be depleted within the next 2 years and that they should move over to IPv6.
ARIN will also start toughening the vetting of requests for additional IPv4 block allocations from the 18th May.
IPv4, or Internet Protocol Version 4, is currently the core addressing system of the Internet but because it has a limited number of addresses, a theoretical maximum of approximately 4.3 billion (232), we are in danger of running out. IPv6, the only real contender to IPv4, is able to address 2128 addresses, or approximately 252 addresses for every observable star in the universe.