Black holes of Internet

The internet is not servers, clouds, wires, routers or code bases. They are agreements about how data flows to and from any one end point and any other. In case of Internet black holes, a University of Washington research project that tracks down a places where packets vanish, either on purpose or by accidentally misconfiguration.

Even TCP/IP delivery a failure back to the sender via ICMP, traffic destined for such addresses is often just dropped. Most firewalls can be configured to silently discard packets addressed to forbidden hosts or ports, resulting in small or large "black holes" in the network. This can be done on purpose for a variety of reasons but the most often used is for security reasons.
Hubble, the monitoring system uses approximately 100 PlanetLab sites around the world to ping various IP prefixes in search of black holes. It uses a hybrid monitoring approach which conducts passive analysis on Border Gateway Protocol feeds and launches targeted active probes when it detects potential problem areas.

The researchers say that Hubble can detect 85 percent of all Internet reachability problems. Most of the network problems detected by Hubble are partial reachability failures, which means that sites can be accessed from some locations but not others.