How come solicited node multicast addresses use only 24 bits of the host's IPv6 address? It looks like there is space for many more; 64 more at a pinch. Using more bits from the host address would decrease even further the likelihood of two nodes sharing the same SNM address.
See RFC 4291, section 2.7.1, page 16.
I can see why at least one bit is needed to discriminate between
all-nodes and SNM addresses[1], but why use so many bits (8)?
The only plausible theory I have is that after 8 bits of "ff" and 16
bits of ethernet multicast prefix (0x3333) are factored in, there is
only room at layer 2 for 24 more bits, so there is no point having more
bits in layer 3. 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).
Regards, K.
[1] All-nodes L2 multicast 33:33:00:00:00:01
SNM L2 multicast of 2001:db8:100:200::1 33:33:ff:00:00:01
SNM L2 with 32 bits instead of 24 bits 33:33:00:00:00:01
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Karl Auer ([email protected]) +61-2-64957160 (h)
http://www.biplane.com.au/kauer/ +61-428-957160 (mob)
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