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

Cisco Systems is helping enterprises prepare for the migration to IPv6 (Internet protocol version 6) from IPv4, which most of the world runs on today.

FORBES: Uncle Sam Gives Cisco A Boost With New Internet Protocol

The protocol then automatically sets up a tunnel over IPv4 to carry communications to other users with IPv6 addresses.

CNN: New protocol sneaks IPv6 traffic over the 'Net