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Why bother plugging in the last 24 bits of the sender's unicast address into the solicited-node multicast address while all-node multicast address can achieve the same thing? In my view, this move is redundant as the requested address is already include in the target address of the packet.

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Why would you want to interrupt every host the way ARP does? By using a solicited node multicast, it probably only interrupts the target host, rather than every host on the LAN. That is a great improvement over ARP, which would have used multicast had it existed when ARP was created.

Even if you have the destination address for the packet, you still need to associate the hardware address to the network address in order to build a frame to encapsulate the packet. IPv4 uses ARP to get the hardware address for a given network address, but IPv6 improved that by using NDP (multicast) instead of ARP (broadcast).

See this answer about that.

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  • Possibly hits 2^24 nodes, vs. a "broadcast" hitting possibly 2^64. (no network will ever get to that size, but at least someone was thinking well ahead.)
    – Ricky
    Oct 17, 2021 at 1:11
  • @Ron I see the advantage of solicited-node multicast address now (Even if not knowing the MAC address, one is able to send to (probably) one host only). But with such advantage, isn't it better to plug in the whole 64 bits to the multicast address to achieve a better chance of sending to one host only?
    – random000
    Oct 17, 2021 at 1:20
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    There is a special format for solicited node multicast, so you do not have 32 bits to use.
    – Ron Maupin
    Oct 17, 2021 at 1:24
  • Hosts can get or get not interrupted based on Mac addresses, not ip addresses (I.e. network card decides which frames to accept based on mac). Multicast IPv6 has to be mapped to multicast mac and this mapping can only use 24 bits of address. So, it is impossible to do more specific filtering. This link explains how mapping is done.
    – Effie
    Oct 17, 2021 at 6:09

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