The terms dark internet or dark address refer to any or all of the unreachable network hosts on the internet.
The global reach of the internet is one of its greatest charms. Some parts of the internet — which is a physical collection of computers and other hardware — are difficult or impossible to access either by accident or design.
A study of these lost sites by US researchers has investigated the extent of the "dark internet" and just what causes a net address to fall off the map.
The study found that up to 5% of the internet - potentially 100 million hosts - is completely unreachable.
The internet works because all the computers on it have an individual address.
Routers, devices that shuffle data around the internet, know where to send information because they maintain lists of these addresses.
But occasionally some of these addresses disappear and researchers at Arbor Networks have spent three years finding out why.
Results of a three-year study on internet 'reachability' have confirmed that the web is partitioned and littered with pockets of 'dark web space' which are home to some of the internet's nasties.
But more worryingly, the study found that this dark space is often used as a launch pad for fleeting internet attacks or as a spamming platform.
The most common cause of dark address space is router misconfiguration. Some military sites on the archaic MILNET address blocks also frequently fall into dark addresses. It is also speculated that hackers utilize malicious techniques to hijack private routers to either divert traffic or mask illegal activity.
Arbor found that these short-lived routing activities, like spamming, indicated a misuse of the routing infrastructure.hackers may increasingly be targeting routing infrastructures as a platform for denial of service attacks.
If a router can stake a claim on a block of address space, the rest of the net's infrastructure will simply accept it and route all traffic for that block.
Because routers aren't set up to log such incidents, these dark corners of the web represent pockets of malicious or sinister activity and "intentional misuse and co-option of the internet routing infrastructure"
Arbor also found a large number of SMTP servers, including over 40,000 unique mail sources, a number of which were associated closely with known spamming incidents. These internet nasties work by exploiting inherent weaknesses in the web's routing infrastructure.
The study reveals that all parts of the internet are not equally connected to every other part. By contrast, some networks inadvertently act as gatekeepers for sections of the internet that lie beyond them.