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I have a very small pcap file in which Neighbor Discovery is performed for Global Unicast Addresses.

That is, IPv6 addresses of both neighbors as I can see in Wireshark start from 2001:. Both IPv6 addresses have common first 64 bits so if I understand right they are indeed belong to one network because -- as far as I know -- the prefix is normally 64 in IPv6.

But everything that I've heard of Neighbor Discovery told me that IPv6 addresses for which MAC addresses are searched should be Link Local Unicast (starting with FE80::/10) rather than Global Unicast. So my question is: why are MAC addresses searched for Global Unicast Addresses instead in my pcap and why do two neighbors in one network want to address each other using Global Unicast rather than Link Local Unicast at all?

Thank you for attention.

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But everything that I've heard of Neighbor Discovery told me that IPv6 addresses for which MAC addresses are searched should be Link Local Unicast (starting with FE80::/10) rather than Global Unicast.

That is incorrect. ND is used to get the link-layer address of any address on the same link. That can be a link-local, global, or unique-local address.

A host having only a global unicast address as a destination address for another host on the same link will search for the link-layer address of the global unicast address. The host has no idea what the link-local address of the destination may be, so it cannot search for the link-layer address of an unknown address.

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  • Thank you for the answer. I understood it. But you know, right now I stopped to understand why we need Link Local Unicast at all?
    – JenyaKh
    Dec 26, 2018 at 0:31
  • There are several reasons. It was first needed for IPv4 that didn't have it, so it was later created for IPv4, but it was a kludge. It was built into IPv6 from the start. See this answer for more information.
    – Ron Maupin ♦
    Dec 26, 2018 at 0:37

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