On Tue, 2011-03-01 at 08:21 -0500, TJ wrote: > On Tue, Mar 1, 2011 at 06:58, Karl Auer <[email protected]> wrote: > This is not a satisfying theory because it appears to tie layer 3 > multicast to a specific layer 2 technology (Ethernet, with its 48-bit > MAC addresses). > > Factoring reality in to any decision typically yields better results, > yes?
Sure. > More importantly - what problem are you really trying to solve? Do > you expect to have lots of collisions in this 24 bit space on a given > link? I'm not trying to solve a problem. I'm seeking to understand why a thing is the way it is. At this stage I have received as answers only rhetorical questions and the statement that 24 is enough bits. 24 certainly is enough bits - I'm just wondering why 24 was chosen, as any greater number up to about 100 would also be "enough", and would cost no more to paste into the SNM address. There is space for more (up to 31) of those bits in the underlying layer 2 multicast address (for ethernet at least), which would be "better" than 24 in some senses, though I can see that returns are starting to diminish. So from anyone who actually knows - why 24 bits? And why spend 8 bits of the ethernet multicast address on what appears to be a simple discrimination between the all-nodes multicast address and SNM addresses? For example, it might be easier and faster to check for "all ones" in a whole octet than to test a single bit - that would explain why ff was used as the discriminator instead of just one bit, and the need for that octet would explain why 24 instead of 31 bits were used from the SNM address. But again, that's just a theory, I have no clue as to whether this is actually the case. I repeat - this is not a criticism, it is a simply desire to know. To me, it's an interesting question. Regards, K. -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Karl Auer ([email protected]) +61-2-64957160 (h) http://www.biplane.com.au/kauer/ +61-428-957160 (mob) GPG fingerprint: DA41 51B1 1481 16E1 F7E2 B2E9 3007 14ED 5736 F687 Old fingerprint: B386 7819 B227 2961 8301 C5A9 2EBC 754B CD97 0156
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