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When doing neighbor discovery in IPv6, the last 24 bits of the hosts MAC address is glued on to some link local multicast prefix. I'm wondering why 24 bits? Why not 48, or 32, or 16? What happens if two MACs on the same network shares the last 24 bits of their MAC addresses?

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The reason for the 24 bits (really 23 bits) is the OUI which is assigned by IEEE. They guys who created multicasting went to the IEEE and registered the OUI for multicasting.

The story I heard was that it cost $1000 (a lot of money at the time) to register an OUI with IEEE, but they didn't have all of it, so they went in with someone else. They then divided the OUI into two parts (the reason it is only 23 bits). Multicasting can only use half the OUI addresses because of this. (Apparently, the other guy didn't use his half for anything of note.)

Remember the destination MAC address for multicast has nothing to do with any interface MAC address. A MAC address has 24 bits for the OUI, and 24 bits to identify the individual interface, per OUI. Multicast uses the multicast OUI and the last 23 bits are for the layer-2 multicast group number. It's true that 23 bits are not enough to represent all the layer-3 multicast groups, so that you have multiple layer-3 multicast groups represented by a single layer-2 multicast group. That is a whole discussion all by itself.

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    Really? This sounds ridiculous. Do you have a source? – Benjamin Lindqvist Oct 27 '15 at 14:35
  • I had an instructor who claimed to know the two guys who created multicast. I don't think he knew the third guy who got the other half of the OUI. In any case, it is the only reason I have ever heard of as to why the multicast OUI is only 23 bits instead of 24 bits as most OUIs are. – Ron Maupin Oct 27 '15 at 14:42
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    @BenjaminLindqvist, a simple search turned this up as to why it is only 23 bits: books.google.com/… – Ron Maupin Oct 27 '15 at 14:54
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    @BenjaminLindqvist In networking, as in other fields, a lot of what is, is due to historical accident and not because of a grand design. So yes, it is ridiculous, but true nonetheless. Multicast was an experiment, and the creators didn't imagine it's widespread use. Actually it wasn't widely used until voice and video over IP came into general use. – Ron Trunk Oct 27 '15 at 14:59
  • This is a good answer but not to the question actually asked. Everything is true but almost none of it is applicable to IPv6 ND. – richardb Dec 8 '15 at 20:00

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