As Vadim corrected pointed out in a earlier post, the Internet is a routing hierarchy. This hierarchy is *dynamic* and not static. Hence, a 'Tier 4' (as Micheal suggests) maybe a 'Tier 6' in two years... It is not practical to define numerical based Tiers, any more than it is practical to say 200/2=100, 100/2=50, 50/2=25.... okay lets stop now, cause NANOG defined fractional division 'tier 4' as the end user in a press release (?) and told the world that you cannot divide 25 by 2... *not allowed*. FYI, 'a backbone' is not a completely definable entity. Is it an abstraction. Discussing the exact defination of an abstraction is much like playing with a Rubic's cube, a nice way to kill time.... BTW. Abstract concepts allow for creative thinking, growth, and other conceptual events. Taking abstractions and trying to turn them into strict constructs and structures limits the creative processes, ad infinitum. Oh well, now back to another round of..... "Name that Abstraction". Regards, Tim