The Internet engineering community is adding a feature to IPv6 that the upgrade to the Internet’s main communications protocol was supposed to eliminate. One of the design goals for IPv6 was that it would rid the Internet of network address translation (NAT), gateways that match increasingly scarce public IPv4 addresses with private IPv4 addresses used inside corporations, government agencies and other organizations.
NAT is deployed in routers, servers and firewalls, and it adds complexity and cost to enterprise networks. Internet purists hate NATs because they break the end-to-end nature of the Internet; this is the idea that any end user can communicate directly to another end user over the Internet without middle boxes altering their packets.
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.