Broadband customers and US military systems are the most common victims of an online phenomenon researchers have dubbed "dark address space," which leaves some 100 million hosts completely unreachable from portions of the Internet.

For a variety of reasons ranging from contract disputes among network operators to simple router mis-configuration, over five percent of the Internet's routable address space lacks global connectivity, according to the results of a three-year study by researchers at Massachusetts-based Arbor Networks, to be released Tuesday.

"Popular belief holds that the Internet represents a completely connected graph," says Craig Labovitz, Arbor Networks' director of network architecture. "It turns out that's just not true."

Anecdotal evidence has long hinted at the existence of dark address space, but the researchers shed light on the subject by continuously gathering and analyzing core routing tables for three years. In the end, they found that for much of the Internet, the shortest path between two points doesn't exist.

The most common factors contributing to dark address space: aggressive route filtering by network operators seeking to ease the load on equipment, and accidental mis-configuration. US military sites frequently fall into the shadow zone because they often occupy neglected 'Milnet' address blocks dating back to the Internet's stone age. Why cable modem customers also top the list remains one of the unsolved mysteries in the project, says Labovitz, who describes the research findings as preliminary.

Murky Crime
Despite the large number of hosts that fall into the partitioned space, the phenomenon is generally not noticeable to average Internet users because most Netizens only use a tiny portion of the Net. "Most people access five or ten web sites," Labovitz says.

The study was conducted by Labovitz, Michael Bailey and Abha Ahuja.

In the course of their monitoring, the team also caught the occasional fleeting glimpse of another, more elusive routing anomaly -- one that often comes with a more sinister explanation.

Blocks of Internet address space that are supposed to be unused sometimes briefly appear in global routing tables, and are used to launch a cyber attack, or to send a flurry of unsolicited commercial email, before being withdrawn without a trace.

Dubbed "murky" address space, this works because of the fundamental insecurity of the Internet's routing infrastructure. Under protocols developed during cyberspace's age of innocence, if an Internet router claims that it owns a block of address space, the rest of the Internet will take it at its word, and route to it all the traffic for that address block. "You co-opt one router, and you can create whatever net-block you want and inject it into the global net," says Labovitz.

Arbor Networks' researchers went to the mail logs of a local ISP and compared several thousand unique mail sources with "murky" addresses spotted in their monitoring. They found that 30 of those addresses sprang into existence shortly before sending the email, and quickly vanished afterwards.

Because routers don't normally log such activity, murky address space could hide the full range of antisocial or illegal network behavior, says Labovitz.

In October, a report from Carnegie Mellon's CERT Coordination Center warned that hackers are increasingly compromising routers, and using them to launch denial of service attacks against Internet hosts.

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