Some time ago I was working on IPv6 implementation and in that period I wrote an article about NDP (you can read it here). After a while I received some comments that is not written very well so I reviewed a huge part of it. It looks my english was far worst two years ago that I was really aware of 🙂
In the reviewing process I realised that NDP usage of Solicited-Node multicast addresses was not clearly explained. This is the follow-up article which should explain how and why Solicited-Node multicast address are used in NDP. After all this kind of multicast addresses are there to enable IPv6 neighbor discovery function of NDP to work properly.
Solicited-node multicast address is IPv6 multicast address used on the local L2 subnet by NDP Network Discovery Protocol. NDP uses that multicast address to be able to find out L2 link-local addresses of other nodes present on that subnet.
NDP replaces ARP
As we know, NDP in IPv6 networks replaced the ARP function from IPv4 networks. In IPv4 world ARP used broadcast to send this kind of discovery messages and find out about neighbours IPv4 addresses on the subnet. With IPv6 and NDP use of broadcast is not really a good solution so we use special type of multicast group addresses to which all nodes join to enable NDP communication.