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IPv6 vs. Carrier-grade NAT

The Internet is running out of IPv4 addresses; there's no argument about that. But what is up for debate is whether ISPs will migrate directly to IPv6 to solve this problem, or whether they will embrace alternatives such as carrier-grade network address translation to share the few remaining IPv4 addresses among their new users.

IPv6 Tutorial

IPv6 is the long-anticipated upgrade to the Internet's main communications protocol, which is known as IPv4. IPv4 uses 32-bit addresses and can support 4.3 billion devices connected directly to the Internet. IPv6, on the other hand, uses 128-bit addresses and supports a virtually unlimited number of devices – 2 to the 128th power.

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The Internet's explosive growth and shifting usage patterns demonstrate why we are running out of IPv4 addresses. The Internet had 1.8 billion users as of May 2010, according to Internet World Stats, and many of these users have more than one Internet-connected device such as a computer and smartphone. That's why 4.3 billion IPv4 addresses aren't enough.

Experts predict that IPv4 address space will run out in 2011 or 2012. In April 2010, the regional Internet registries said in April 2010 that less than 8% of IPv4 addresses remain unallocated.

Carriers must decide what approach to take to deal with the inevitability of IPv4 address depletion.

Some carriers, including Hurricane Electric, NTT America, and Verizon, are embracing IPv6. Hurricane Electric and NTT already offer IPv6 service in the United States, while Verizon is running an IPv6 trial for a handful of residential customers in northern Virginia.

Comcast is taking a multi-pronged approach to IPv6, running customer trials of dual-stack IPv6 and carrier-grade NATs at the same time. Comcast is testing a NAT approach it created called Dual-Stack Lite to share one IPv4 address among multiple customers, but it would prefer for the Internet infrastructure to adopt IPv6 instead of another layer of NAT devices. Comcast's IPv6 trials have attracted more than 5,500 customers.

The Internet has a history of adopting mechanisms for stretching the life of an older technology rather than upgrading to a newer one. For example, in 1993, the Internet engineering community created Classless Inter-Domain Routing (CIDR) to slow the growth of the Internet's routing tables and to delay IPv4 address exhaustion. Private address space was created in 1996 to allow network operators to use NAT devices to hide private network addresses behind a single public-facing IPv4 address. So it's possible that ISPs will choose carrier-grade NATs over IPv6 this time around.

The Internet engineering community, however, is urging ISPs and content developers to adopt IPv6 quickly -- before IPv4 addresses are depleted. John Curran, president and CEO of the American Registry for Internet Numbers, says Web site operators have until Jan. 1, 2012 to adopt IPv6 or face performance degradation. Google is a leading proponent of IPv6, and other content providers such as Netflix and Facebook are following suit.

Read more about lans and wans in Network World's LANs & WANs section.

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