Three Internet Standards, one of them historic, RFC 3330, an RFC 3330 erratum, and three Internet Drafts entered the battle over 6.25% of all IPv4 addresses:
RFC 3330 lists special IPv4 addresses, among others 240.0.0.0/4 also known as class E. Allegedly "reserved for future use" on page 4 of the historic RFC 1700, formerly known as STD 2. The notation 240.0.0.0/4 stands for all IPv4 addresses starting with the four bits 1111, hex. F0 is decimal 240.
Page 4 of RFC 1700 actually belongs to an introduction with basic terms copied from RFC 1122, a part of STD 3. In fact RFC 1122 specifies 0.0.0.0/8 and 127.0.0.0/8 as listed in RFC 3330, but not 240.0.0.0/4.
RFC 1122 and the RFC 1700 intro only mention class E, but it is specified in RFC 1112, a part of STD 5. Apparently RFC 3330 got this wrong, RFC 1700 by itself did not register anything at all, it was a snapshot of various IANA registries with references to registrations of all "assigned numbers".
As the future of IPv4 is now before IPv4 is replaced by IPv6 the question is what to do with these 6.25% of all IPv4 addresses "reserved for future use" minus one 255.255.255.255 specified in RFC 1122. Three Internet-Drafts fight over it:
- IANA proposes to keep class E as is, because RFC 1700 said so.
- CISCO proposes to unreserve these 268,435,455 IPs reserved in RFC 3330.
- A third draft suggested to reserve these IPs for private use updating RFC 3330.
Ignoring the expired third draft as bad idea it will be interesting how the IETF manages to free this huge pool of IPv4 addresses while the rest of the world is busy to upgrade their hard- and software to IPv6. Apparently RFC 1112 does not do anything with class E apart from reserving it, it is about class D 220.127.116.11/4 multicast addresses. Or as RFC 1812 puts it:
The Class D (IP Multicast) and Class E (Experimental) address spaces are preserved, although this is primarily an assignment policy.
Update 1, it's a bit more complex: RFC 988, a predecessor of RFC 1112, created the 18.104.22.168/4 multicast addresses as class D out of the in RFC 960 reserved 22.214.171.124/3 block, the remaining 240.0.0.0/4 block got the name class E in RFC 990 obsoleting RFC 960. After that RFC 997 updated RFC 990 and listed the "Internet numbers" including class E, while RFC 1010 obsoleted RFC 990 and listed the "assigned numbers". RFC 1010 was a predecessor of RFC 1700 over many steps, RFC 997 was the predecessor of RFC 1166. In other words, RFC 1112 specifies class D, and RFC 1166 explains the remaining class E. RFC 1166 is only "informational", that's good news from the procedural side.
Update 2, a fourth draft was published: T. Savolainen proposes to use class E for hosts indicating that they can handle this , e.g., for dial-up connections.