In a word: Growth. The number of Internet-connected users, devices and applications are growing at such a rapid clip that the pool of available addresses for the original version of the Internet Protocol, known as IPv4, is being rapidly depleted. IPv4 uses 32-bit addresses and can support 4.3 billion devices connected directly to the Internet. The replacement protocol, called IPv6, uses 128-bit addresses and provides such a vast number of addresses that it can only be expressed mathematically: 3.4 x 10 to the 38th power. A few skeptics question the need for IPv6. But most Internet policymakers and network technology experts assert that the shift to IPv6 is inevitable, and the time to move is now.