[iofj]

Well it's pretty much the addresses you start using when you've run out of IPv4 addresses everywhere else.

A very popular example of this is 100.64/10, but one can find little bits here and there. Plenty of providers don't just use that range but 1/8 is pretty safe to use.

There are even posts on networking mailinglists about tests for using the multicast ranges (multicast doesn't work* anyway and is now widely considered a "never gonna happen" design). Leave 224.0.0.0/24 alone and you can pretty much use the rest of 224.0.0.0/4. Also, most of broadcast is fine to use on most networking equipment.

* of course, locally within a network it does work for a very small number of multicast streams (certainly doesn't work for 2^28 multicast streams as designed, so in ipv6 they upped the number of available multicast channels to 2^120)

[duskwuff]

1/8 is absolutely not safe to use. There are many real IP addresses assigned in that range; for instance, 1.5/16 is assigned to a Japanese ISP.

[iofj]

http://www.bgpmon.net/issues-with-allocating-from-1-0-0-08/