Roger's odd programming question of the week

[Kurt Wall]

On Fri, Sep 02, 2005 at 02:12:47PM +0200, Roger Oberholtzer took 32 lines to write:
> 
> If a network server app is programmed to multicast on some network
> address, would a client on the same network simply listening for
> broadcasts see the multicast packet?

Not necessarily. The client has to have joined the multicast group. In
practice, I think this is the case (as you can see by pinging the
all-hosts multicast group (224.0.0.1) because multicast capable
interfaces are required to join the all-hosts group and (at least in the
Linux TCP/IP stack) multicast is usually enabled..

> Conversely, if a network server wrote to a broadcast address, would a
> client on the same network that has joined a multicast group see the
> broadcast packet?

Yes. A broadcast address includes all of the addresses on a subnet.
That one or more clients have joined a multicast group won't really
impact the deliver of broadcast datagrams.

> I guess my basic question is: is a multicast a subset of a broadcast, or
> is it something different? I have read a few discussions on this that
> imply (based on packet headers) that this may be the case. But, as
> always, I thought someone here may have an opinion.

Heck. Give me a while, and I'll have two different opinions. :-)

Kurt
-- 
God doesn't play dice.
		-- Albert Einstein