Also in the letter, ARIN advised address holders to begin planning for IPv6 adoption. In particular, it said they should make their publicly accessible resources, such as Web and e-mail servers, accessible via IPv6.

IPv4 depletion is most dire in parts of the world with fast-growing populations of Internet users, such as China and India, but that doesn't mean U.S. enterprises will dodge the bullet completely, Passmore said. Once users start switching over to IPv6, any company that wants to reach those users will need to support the new protocol. However, IPv4 addresses within an organization will still be usable. The two protocols can coexist in a network.

"This does not mean that typical large enterprises are going to have to go out and re-number their networks," Passmore said. There's no wholesale change on the horizon even if IPv4 addresses run out when ARIN believes they will. "Even two years from now, the IPv6 traffic will only be a tiny fraction of IPv4," Passmore said.

IPv6 adoption has been slow despite warnings about address depletion going back a decade or more. A by Arbor Networks and the University of Michigan, released last August, showed that IPv6 traffic represented less than one one-hundredth of one percent of all Internet traffic.