Re: understand multicasting from the client/host perspective .


Mike, you are the best!

I think IGMP has to bee used for sending to hosts in a specific
multicast group, my teacher told me that ...

I think my remaining question is on the conversion of the last leg
multicast IP to MAC, why that is required, and how it is being used (by
the switch?)?

I think we are getting there ... :-)

HeadsetAdapter.com wrote:
"April" <yinzhang57@xxxxxxxxx> wrote in message
news:1151341027.363421.64350@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

Oh by the way, try this specifi one ...

The multicast MAC part is where I have difficulty to understand well ..

like why this is needed? As the sender, it seems to me it only needs
to send to an IP ... I can think this signifies the frame is multicast,

and somehow the switch can relate the multicast MAC to the MAC of a
host that joined the group, but not sure this is all it about?

Thanks in advance .


OK. Get back to the "4th grade language" :-)

What's a difference between IP address and MAC address. Let's say, the IP
address is your "Public Mailing Address", for example, 1234 Main Street,
Room 234, Best Town, CA, 95001. People send you a correspondence to that
address. But they don't care what color is your mailbox, what row is it
located, etc. But mailmen does. He knows that your mailbox is third one on
the fourth row, right over the corner.

Now, let's say some Men's magazine sends a preview issue to all male, who
live in this house. They just sent a bunch of magazines to your house, and
mailmen will place them into the boxes, where male tenants live. That will
be "multicast".

But if mailmen is not aware, who lives where, it will place the magazine in
each mailbox. He "converts" a "multicast" (Men's magazine) to "broadcast"
(what if there is a men in the house...). Publisher's intent was to deliver
the issue to male population only, but since mailmen was not aware, he
delivered them to every mailbox.

And another situation - your town wants every person in the town to receive
an invitation. They send a big box of postcards to a local post office. And
mailmen instructed to put the postcard into each and every mailbox. So, he
does not care, who lives there - he just delivers a mail.

And now back to "technical language"... Each network device has it's own MAC
address. Your switch knows, which MAC address available thriugh which port.
So, in case on unicast, it knows, that specific freame should be delivered
to the specific port. Also there is a "Layer 2 broadcast" MAC address (often
named Ethernet broadcast). Any frame with the bradcast MAC address as the
destination, will be delivered to each and every port on the switch. And
third "group" of MAC addresses is defined to be a Multicast MAC addresses.
You can look at this MAC address as on "conditional broadcast". It will
reach the port only if certain condition met (port subscribed to specific
Multicast group). Switch does not care what was the IP address of the group,
nor what was the IP address of the sender. It knows, that it has to deliver
the frame based on the destination MAC address.

Good luck,

Mike
----
www.headsetadapter.com
Headset Adapter for a Cisco IP Phone

.

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