Sean Donelan <SEAN@SDG.DRA.COM> writes:

What I was trying to postulate, unsuccessfully, there is no such thing as an universal, optimal hierarchical addressing scheme. I thought I had chosen examples from the opposite ends of the spectrum. I guess I wasn't extreme enough in my examples. Perhaps I should have used the ISBN hierarchy, a combination of language group, country and publisher prefix. I'm going to publish a million books, so I should get a 'big' publishers prefix.

You are confusing transport address (describing a location in a topology) with an object name (a book, a computer, a process running on a computer). An object name like an ISBN does not need to be hierarchical because they do not describe discrete locations. Introducing hierarchy improves managability and efficiency of databasing. Many hierarchies can be imposed on object names. IPv4 addresses and anything like them need to be hierarchical in sufficiently large networks because they do describe the location of one or more objects, and because flat addresses are known not to scale in large networks. Hierarchical routing is the only known means of scaling IP addresses as they exist now, and therefore the only hierarchy that can be imposed on IP addresses is strictly topological. The canonical object-describing database that is the roughly the analogue of the ISBN database is the DNS. (I hate analogies I hate analogies I hate analogies please don't use them). The DNS is also manifestly hiearchical, and that hierarchy introduced efficiency compared to the former flat ARPA namespace. Note that the DNS works with suffixes rather than prefixes, which is a cosmetic difference unless one is interested in doing binary sorts or tree-based searces, and that the DNS is variable-length, which is not a cosmetic difference from the ISBN. If you are a big organization and plan to have lots of objects you need a sufficient swathe of DNS names to describe them all. These in turn should resolve into LOCATORS which describe where in the Internet topology (as opposed to the corporate topology or the geographical topology) the objects can be reached. For example, one hierarchy of Internet object names is clock.org. Although cesium.clock.org and solar.clock.org are siblings in that hierarchy, they have very different IP addresses because they are located in different parts of the Internet topology. They should NOT have the same IP prefix as any attempt at that would introduce unnecessary inefficiency into the routing system. Remember: at each level of naming there can be different and completely disjoint hierarchies. There are scalability implications in all of them, most notably when the names used have size limits or are distributed non-hierarchically (like ethernet addresses or the COM domain). However, the important thing is that when a name is used as a LOCATOR in a topology, in order to be scalable that name must be related to that topology and in large networks must lend itself to aggregation in order to reduce the amount of information needed to have that locator be used throughout the network. Sean.