IPv6 ND - What am I missing here?

iamme4eva

Hi,

I've been googling and I'm still a little confused - I'm obviously missing a piece of this jigsaw...

IPv6 does away with broadcasts. We all know that. There's no such thing as ARP. It's now neighbour discovery.

What I don't get is how it prevents a layer two broadcast? If we want to communicate with another host, we go down the protocol stack through IPv6 and come to regular old ethernet - so we need a destination MAC address to address the frame to. So we send a neighbour solicitation to the host, to the multicast group ff02:0:0:0:0:1:<last bit of MAC>. However, that NS needs to be put in a frame addressed to .......................................?

And this is where I get lost. Surely, while the NS is going to a multicast IPv6 address, it still needs to go to a broadcast MAC address?

I've labbed it and I see that the destination MAC address is 3333.FF00.0002. I've no idea where this has come from.

Sorry if this is a dumb question - I'm obviously googling the wrong keywords!

iamme4eva

I think I answered my own question, when I stopped worrying about neighbour solicitation and just looks up IPv6 multicast.

For IPv6 Multicast addresses, the Ethernet MAC is derived by the four low-order octets OR'ed with the MAC 33:33:00:00:00:00, so for example the IPv6 address FF02:DEAD:BEEF::1:3 would map to the Ethernet MAC address 33:33:00:01:00:03.

powmia

Yep, there are multicast mac addresses too