Sometimes a picture is worth a thousand words. BBN is the random scatter of green in the middle (early ARPANET). Sprint is the organized star topology in purple near the top. AOL is a gray disconnected island in the lower center. There is little correlation between this network connectivity graph and physical geography, except for a clustering of Pac Rim connectivity.
This technique can also see the network “lights go out” during wartime bombing raids.
In a nutshell, they use a modified hacker trick of sending a storm of IP packets out randomly across the network. Each packet is programmed to self-destruct after a delay, and when this happens, the packet failure notice reports back the path the packet took before it died. To visualize this sea of data, Ches applied place & route software from the semiconductor CAD industry to untangle the hairball of data and spread it out in a 2D map that humans can easily absorb. In these maps, one can see security gaps and unknown network connections. (disclosure: we invested them when they spun out of Bell Labs)