all host multicast

[dan731028]

I'm on an IPv6 crash course and have ran across a term that I'm having a hard time defining.  I was wondering if anyone could explain what an all host multicast is and how that is different from a broadcast?  If all nodes on the local link get the multicast, isn't that a broadcast? 

[desc]

Yes, it sounds like a kind of broadcast, but it is not intended to be broadcast, as it usually only works on a scope 1 (node-local), or 2 (link-local) network and it is not depending on a broadcast domain (respective an unicast IPv4 range).

[dan731028]

That makes some sense.  I guess I need to get a better grasp of scopes.

[jimb]

It's a multicast which addresses all hosts on a LAN.  There's also an "all-routers" multicast.

[cholzhauer]

What would something like this be used for?  To be honest, I've never had the need to use an IPv4 multi cast address, much less a V6 one

[jimb]

What would something like this be used for?  To be honest, I've never had the need to use an IPv4 multi cast address, much less a V6 one

Any application where you want multiple hosts to receive the same packets.  For instance, streaming video or audio broadcasts.  Disk imaging to multiple clients.  Or router or neighbor solicitation, both which make use of multicast addresses which routers and all nodes (respectively) are required to subscribe to.

IPv6 defines certain well known multicast addresses for purposes similar to what broadcasts are used for under IPv4.  IPv6 basically made broadcasts redundant, since everything broadcasts are used for under v4 has a multicast address defined which is usable for the equivalent purpose.  For instance, ARP under v4 uses the all-ones broadcast address, and ND under v6 uses the link-scope all-nodes multicast.  (Note: this isn't to say the v4 doesn't also make use of multicast addresses.  It does, for lots of stuff.)

http://tools.ietf.org/html/rfc2375 <- list of well known ipv6 mcast addresses.

Anyway, shorter answer, you use multicast addresses every time you turn up an IPv6 interface on a LAN, since it's required for ND, etc, etc.

[woosingwoo]

it is not depending on a broadcast domain.

Is that means if we put two hosts on different vlans in same switch, the hosts still can ND each others?

[cholzhauer]

it is not depending on a broadcast domain.

Is that means if we put two hosts on different vlans in same switch, the hosts still can ND each others?

I hope someone will correct me if I'm wrong, but no, I don't think this is the case

[jimb]

There are different scopes of multicasts (for instance, I believe ND is limited to link scope).  Some can traverse routers and LANs, others can't.  Ethernet multicasts will typically flood a LAN segment much as a broadcast would unless the switch fabric employs IGMP and/or MLD snooping.

If you really want to know how it all all works, look into IGMP (IPv4's multicast membership signaling scheme), MLD (IPv6's), and some of the multicast routing protocols like PIM (although my knowledge of this might be dated now.. haven't really looked into that stuff for a while).