Kevin Poulson on dark address space: parts of the Internet that can't find a route to each other. Result is by Internet researcher Craig Labovitz, although I can't find this paper. Some hints by Poulson that this could be related to folks hacking Internet routers to make safe spaces for themselves. Fun stuff. [RobotWisdom].
Way back when, the Internet was only 30 hosts wide. That is, the time-to-live field on packets in the common TCP/IP implementation was set to 30. If two hosts had more than 30 hops on their route, they couldn't talk to each other. The Internet grew bigger than a diameter of 30 sometime in 1992 or so, and all those TCP/IP stacks had to be updated. I think most stacks now set the TTL to 255, the maximum.