The internet was built on version 4 of the Internet Protocol (IPv4) which has an upper limit of about four billion addresses.

BBC: Net approaches address exhaustion

The problem is that the original version of Internet Protocol (IPv4), on which the bulk of the Internet has been built thus far, has a fixed capacity of 4.3 billion IP addresses.

FORBES: The Internet Gets Its First Global Upgrade: Happy World IPv6 Day

This expansion is necessary because the current number of addresses under Internet Protocol version four (IPv4) is gradually being exhausted.

WHITEHOUSE: Office of Science and Technology Policy Blog