This graph was produced with vulnerability data sniffed by the Passive Vulnerability Scanner (PVS) from Tenable Network Security and AfterGlow. A blog entry detailing this graph is located at Tenable's blog. The PVS sniffs vulnerability data such as client and server vulnerabilities, but also network 'trust' relationships such as which machines connect to a server on port 22. In the above graph, a host with an arrow to another host represents that the PVS has observed at least one network connection between those systems.