solicited-node multicast address

The IPv6 multicast group, ff02::1:ff00:0/104 plus an address's last 24 bits, that Neighbor Discovery asks instead of broadcasting.

Where ARP broadcasts to every host, IPv6 has no broadcast and sends address resolution and duplicate address detection to the solicited-node group of the target, which every node joins for each of its unicast and anycast addresses. Only the nodes sharing those 24 bits are interrupted.

The group is built by appending the low-order 24 bits of the address to ff02::1:ff00:0/104, which gives 2^24, or 16,777,216, possible groups. For 2001:db8::200:5eff:fe00:5301 the bits are 00 53 01 and the group is ff02::1:ff00:5301. Its link-local twin, fe80::200:5eff:fe00:5301, falls in the same group, by intent: addresses that differ only in their high-order bits share one, so a node joins fewer groups.

On Ethernet an IPv6 multicast is sent to a MAC address made of 33:33 and the last four octets of the group, here 33:33:ff:00:53:01, whose group bit marks it as multicast. Nodes join with MLD; a switch without MLD snooping still delivers the frame to every port, so the gain is fewer interrupts on other nodes rather than less traffic on the wire.

Duplicate address detection uses the same group: before using a tentative address a node joins its solicited-node group and sends a Neighbor Solicitation to it from the unspecified address. In Python's ipaddress the group is the prefix ORed with the address masked to 0xFFFFFF; the IPv6 Toolkit classifies the result as link-local multicast but does not derive it.

Also known as: solicited-node address, solicited node multicast, solicited-node group, ff02::1:ff00:0/104

Sources

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