Worked example: derive the solicited-node multicast address for the GUA 2001:db8:acad:1::1.

ff02::1:ff00:1 — take the fixed ff02::1:ff prefix and append the last 24 bits of the unicast address (00:00:01).

Step by step:

Unicast (expanded last hextets): ...:0000:0001
Last 24 bits of the address    = 00 00 01   (hex)
Prefix                         = ff02:0000:0000:0000:0000:0001:ff00:0000  →  ff02::1:ff00:0/104
Append last 24 bits            = ff00:0001
Result                         = ff02::1:ff00:1

Second example (random Interface ID):

GUA ending in ...a5bb:66e1  →  last 24 bits = bb:66e1
Solicited-node              =  ff02::1:ffbb:66e1

Why it matters: Neighbor Discovery (the IPv6 replacement for ARP) targets this address instead of a broadcast, so only hosts whose last 24 bits match — almost always just the one intended host — have to process the request. The NIC can even filter it in hardware by the mapped Ethernet multicast MAC.

Go deeper:

• RFC 4291 §2.7.1 — Solicited-Node Multicast Address — the exact prefix-plus-low-24-bits rule this example applies.