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A Solicited-Node multicast address is an IPv6 multicast address used by the Neighbor Discovery Protocol to verify whether a given IPv6 address is already used by the local-link or not, through a process called DAD. This allows NDP to assign IPv6 addresses to hosts using SLAAC without the risk of assigning addresses already in use. The Solicited-Node multicast addresses are generated from the host's IPv6 unicast or anycast address, and each interface must have a Solicited-Node multicast address associated with it.
NDP sends out a Neighbor Solicitation message to the Solicited-Node multicast address of the IPv6 unicast or anycast address it plans to assign using SLAAC, and if a host is present in that group, it will respond with a Neighbor Advertisement message , and NDP will know that the IPv6 unicast or anycast address it is trying to assign is already in use.
A Solicited-Node address is created by taking the least-significant 24 bits of a unicast or anycast address and appending them to the prefix ff02::1:ff00:0/104.