What about those that are assigned and used but not [currently] visible on the public Internet [i.e., are on other internets]?
Indeed! On Henk's slide number 5 he states: "Each AS wants to be able to send traffic to any other AS" This is NOT true. Many ASes explicitly do *NOT* want to send traffic to any other AS. They only want to send traffic to customers, vendors or business partners of some sort. In other words, there are many so-called extranets which are basically internets built using exactly the same technology as the Internet however with more restrictive interconnect policies. One way to visualize this is to imagine the Internet as a cloud. At the core of the cloud are the core providers and at the edge of the cloud are the end user organizations, many of which appear to be singly homed. However, hidden behind this edge is a thin layer which represents a private internet. It also connects many networks but it does *NOT* exchange traffic with the public Internet. All the networks connected to these private internets are also connected to the public Internet but they implement strict traffic separation policies internally. In some cases, this is an air gap but these days it is often a bunch of firewalls. In the 24/7 connected world of the 21st century there is a lot of growth in these internets that wrap around the Internet but don't exchange vital fluids with it. --Michael Dillon