IPv4 address depletion

Prompting the development of NATs for IPv6 is the current estimate that the Internet will run out of IPv4 addresses in 2011. IPv4 uses 32-bit addresses and can support 4.3 billion devices -- not enough for the world's 6.5 billion people and all the Internet-connected PCs and cell phones they own.

IPv6 uses 128-bit addresses and can support a virtually limitless number of devices -- 2 to the 128th power -- connected directly to the Internet. IPv6 also has built-in security and network management enhancements. IPv6 backers have long touted the removal of NATs from the Internet as one of the key reasons for migrating from IPv4 to IPv6.

Despite its benefits, IPv6 has been slow to catch on outside of Asia, where IPv4 addresses are scarce. In the United States, the federal government is leading the way to IPv6 adoption.

Alain Durand, chair of the IETF's Softwires working group and a long-time IPv6 proponent, says the IETF must rethink how IPv6 will be deployed because of looming IPv4 address depletion. Durand is director of Internet governance and IPv6 architecture in the Office of the CTO at Comcast.

"The original master plan 15 years ago was that everybody was going to deploy IPv6, and all the devices would be both IPv4 and IPv6 dual stack. The whole universe would be this way long before IPv4 addresses ran out. Well, it didn't happen that way," Durand says.

The pressure for the IETF to develop NATs for IPv6 is coming from carriers and early IPv6 adopters such as the Chinese government.

When IPv4 addresses are depleted, carriers will give their new customers IPv6 addresses. But all of the PCs, printers and gaming systems owned by these customers won't be upgraded to IPv6. That's why carriers need a mechanism to translate between IPv4 and IPv6 addresses.

Both Comcast and Free, a French ISP, are considering rolling out NATs as part of their IPv6 implementations. Comcast has proposed to the IETF a NAT-and-tunneling combination called Dual-Stack Lite, while Free has proposed a mechanism that the carrier used to deploy IPv6 to 1.5 million consumers in France.

Durand's proposal includes traditional IPv4 NATs housed inside carrier networks along with IPv6-to-IPv4 tunneling at the edge of the network. Durand says this approach is the only realistic alternative to multiple layers of NATs translating between private IPv4, public IPv4 and public IPv6 addresses.

"We have found a way to combine tunnels and classic IPv4 NATs to provide IPv4 services to our customers after the exhaustion of IPv4 addresses," Durand says, declining to comment on the irony of the situation. "That's what really matters."

The Chinese government used NATs to interoperate between the Chinese Education and Research Network (CERNET), which is IPv4-only, and CERNET2, the next-generation Chinese Internet backbone that is IPv6-only.

Baker says the Chinese have been using a NAT approach dubbed IVI for about two years. "That makes it a strong contender in a world of rough consensus and running code," Baker says of IVI.