The coming shortage of Internet Protocol addresses prompted the American Registry for Internet Numbers (ARIN) on Monday to call for a faster migration to the new Internet Protocol, IPv6.

The current version, IPv4, allows for more than 4 billion (2^32) Internet addresses. Only 19% of the IPv4 address space remains. Somewhere around 2012-13, the last Internet address bloc will be assigned and the Internet will be full, in a manner of speaking.

"We must prepare for IPv4's depletion, and ARIN's resolution to encourage that migration to IPv6 may be the impetus for more organizations to start the planning process," said John Curran, chairman of ARIN's Board of Trustees, in a statement.

IPv6 promises some 16 billion-billion possible addresses (2^128).

IP numbers are used to route traffic around the Internet. They're not the same thing as Internet domain names, which get mapped to IP numbers through the Domain Name System (DNS) because it's much easier to remember "Amazon.com" than "72.21.203.1."

"Unless action is taken now, a quiet technical crisis will occur, not unlike Y2K in its complications, but without a fixed date or high level public attention," wrote Stephen M. Ryan, a partner at McDermott Will & Emery LLP and ARIN's general counsel, and Raymond A. Plzak, CEO and president of ARIN, in a forthcoming policy paper.

Ryan and Plzak foresee potential legal problems arising as address scarcity leads to a new black market in IP numbers.

IP numbers are not, like Internet domain names, seen as property by U.S. courts as a consequence of Gary Kremen's litigation to recover the Sex.com domain. In the course of that dispute, Kremen in 2001 was awarded the assets of an ISP that defendant Stephen Cohen had acquired, which included IP numbers.

Kremen then engaged in litigation with ARIN to obtain his IP numbers without signing the ARIN registration service agreement. In essence, he claimed ownership rights in the IP numbers rather than the more limited set of rights governed by ARIN's contract. The U.S. District Court in San Jose, Calif., rejected Kremen's claim last year, upholding ARIN's regulatory power over IP numbers.

But a number of companies, organizations, and individuals hold IP numbers that predate ARIN's arrival on the scene in 1997. Holders of IP address blocs awarded during the Internet's early days may be sitting on a gold mine. Because they're not bound by an ARIN contract, they're theoretically free to sell their IP numbers. They haven't done so because, among other things, there's no money in it at the moment. But if the IPv6 migration continues to lag and IP addresses become scarce, holders of legacy IP address blocs could find it profitable to sell their numbers.