An article on slashdot mentions this news from Tech Review: according to relatively new measurements a lot of the IPv4 addresses currently allocated are not actually in use.
The census, carried out every quarter since 2003 but only recently published, is the first comprehensive map of the Internet since David Smallberg, then a computer-science student at the University of California, Los Angeles, canvassed the Internet’s first servers–all 300-plus of them–following the switchover from the ARPANET in early 1983.
The article itself is a bit light on details, and forgets to link to the actual paper they are quoting from, but the general information presented is accurate and shows that there are a lot of IP addresses that are allocated to different organizations, but not actually in use. If you don’t want to read the entire paper, the researchers also have a blog and a variety of maps.
The article and the underlying research raise a valid point: if IPv4 addresses are getting scarce, shouldn’t it be reclaimed from people that hold large address blocks that are unused? This might get a bit complicated if every address needs to be accounted for, but there is a place we can start. There are about 40 /8’s (nearly 700 million) IP addresses that are allocated, but don’t appear in the internet routing tables. That means that these addresses are unreachable for at least a large portion of the internet; in other words, definitely unused. If you combine that with the new census done at ISI that says about 100 million new addresses are needed per year, this would buy a minimum of 5 extra years in which to complete the transition to IPv6.