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The Internet engineering community working on IPv6 is considering reintroducing NAT - one of the very features that the upgrade to IP was meant to eliminate.

IPv6 was planned to rid the Internet of network address translation (NAT), the scheme used by gateways to match increasingly scarce public IPv4 addresses with private IPv4 addresses used inside corporations, government agencies and other organisations.

NAT is deployed in routers, servers and firewalls, and it adds complexity and cost to enterprise networks. Internet purists hate NAT because it breaks the end-to-end nature of the Internet. But because it has taken so long to migrate the Internet from IPv4 to IPv6 - IPv6 is 10 years old and not yet widely deployed - and because IPv4 addresses are running out faster than Internet users are able to roll out the preferred method of IPv4-to-IPv6 transition known as dual-stack operation, the Internet engineering community has come to the conclusion that it must create special NAT devices to translate between IPv4-only and IPv6-only hosts.

"When the chips are down, NATs may be the only way we are going to get IPv6 added to the Internet," says Fred Baker, a Cisco Fellow who was chair of the IETF when IPv6 was designed. "If we have IPv4-only and IPv6-only networks, both of which we have now, NATs are the only way they will connect."