This is essentially correct. The apparent conceptual difference is that a variable length address looks more like source routing. The end system owns only a small part of the total address; the rest is the network portion, fashioned to seem like a source route. Depending on how the address is interpreted, the division of the network portion into routing steps will be specified in advance or will be interpreted at each step. The latter alllows gradual evolution of the network by consolidating small switches into larger ones. The transition to CIDR within IPv4 accomplished pretty much the same thing and had the same benefits. Nonetheless, 32 bits just isn't enough. The only way variable length address would have provided a smooth transition is if there had been room to increase the length of the address after some years of use. Well, if we had left room in the address field for more than 32 bits, we'd have had the same advantage. But we didn't. Steve On Feb 15, 2012, at 4:10 PM, Stephen Sprunk wrote: > On 15-Feb-12 08:42, Dave CROCKER wrote: >> As I recall, there was essentially no experience with variable length >> addresses -- and certainly no production experience -- then or even by >> the early 90s, when essentially the same decision was made and for >> essentially the same reason.[1] >> >> It's not that variable length addressing is a bad idea; it's that it >> didn't get the research work and specification detail it needed, for >> introduction into what had become critical infrastructure. What I >> recall during the IPng discussions of the early 90s was promotion of >> the /concept/ of variable length addressing but without the >> experiential base to provide assurance we knew how it would operate. > > The problem with variable-length addressing that, in practice, one needs > to specify a maximum length. The result, therefore, is that you don't > have variable-length addresses at all but rather fixed-length addresses > with a shorthand encoding for unused bits. > > S > > -- > Stephen Sprunk "God does not play dice." --Albert Einstein > CCIE #3723 "God is an inveterate gambler, and He throws the > K5SSS dice at every possible opportunity." --Stephen Hawking > > > _______________________________________________ > Ietf mailing list > Ietf@xxxxxxxx > https://www.ietf.org/mailman/listinfo/ietf _______________________________________________ Ietf mailing list Ietf@xxxxxxxx https://www.ietf.org/mailman/listinfo/ietf