One example I heard was a generic financial exchange connected to perhaps a hundred other companies. Those companies also connect to the Internet but the exchange itself does not. It's valuable for the exchange to use addressing which will not conflict with any of its customers' RFC1918 use or overlap any Internet destinations they want to access.
Sounds like SFTI in New York http://www.nyse.com/technologies/sfti/1223635951074.html In turn, SFTI is connected to the Radianz global IP network which allows financial industry companies in other countries to access the NYSE services on SFTI. And the Radianz global IP network has over 15,000 sites connected to it in some 200 countries. Probably all of the companies connected to Radianz also have an Internet connection, but nobody passes packets between Radianz and the Internet. Radianz is an example of a COIN (Community of Interest Network). Outside the Financial Services industry there are similar COINs in the air traffic industry (SITA) and the auto manufacturing industry. If you diagrammed these COINs on a typical Internet diagram, they would be a thin layer, one AS thick, wrapped around some portion of the cloud's perimeter. Invisible to most because they connect but do not exchange transit traffic. Zoom in an look at ASCustomer which peers with three ISP ASnumbers and also peers with ASRadianz. But the traffic from ASRadianz is controlled by firewalls and internal routing in ASCustomer so that it only goes to the trading workstations, while the Internet traffic is allowed pretty much everywhere. You could make various biological analogies such as the specialised layers of human skin cells or the micturating membrane in amphibians. --Michael Dillon