If you've received even a day of basic networking training, odds are that you're familiar with the original classful structure of the IPv4 Internet (which still appears on the CCNA, despite having been obsolete for as long as I've been around). The range of all possible IPv4 addresses was originally segmented into five classes, A through E. A, B, and C were used for general unicast assignments, class D was designated for multicast addressing, and class E was set aside for "experimental" use. In fact, that chunk of of about 268 million IPv4 addresses is still designated by IANA for "future use" today.
With the remaining available IPv4 address space due to run out some time next year, it would seem that now might be the time to break out the old class E addresses IANA has seemingly been saving for a rainy day. Unfortunately, it's not all that easy.
Two Internet drafts have been proposed for allocating the former class E space (240.0.0.0/4) for general use. A draft titled Reclassifying 240/4 as usable unicast address space was put forth by some engineers at Cisco, but it is expired as of September 2008. A similar draft, Redesignation of 240/4 from "Future Use" to "Private Use", calls for the 240/4 space to be allowed at least for private use, but it too is expired.
Why does there appear to be so little support for the initiative? From the former of the two drafts mentioned above:
At the present time, most IP implementations consider any IP address in the range 240.0.0.0 through 255.255.255.255 to be invalid as the source or destination of a datagram. The check for such "illegal" addresses may occur in many places, including at datagram receipt, before IP datagram transmission, when an IP address is assigned to a network interface, or even by router and firewall configuration parsers.
Iljitsch van Beijnum offers some additional insight in an article from The Internet Protocol Journal:
The class E space has 268 million addresses and would give us in the order of 18 months worth of IPv4 address use. However, many TCP/IP stacks, such as the one in Windows, do not accept addresses from class E space and will not even communicate with correspondents holding those addresses. It is probably too late now to change this behavior on the installed base before the address space would be needed.
Had an initiative to reclaim the 240.0.0.0/4 space for general use been made say ten years ago, it would have had a good chance at succeeding. Unfortunately, at this late stage in the IPv4 game, reclaiming the space would necessitate an incredibly broad Internet-wide initiative, and yield only a modest return. Besides, we don't need another reason to keep IPv4 around just a little bit longer. IPv6 is here: start using it.