Harold Ritter Wed, 11/28/2007 - 13:03
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The same source can indeed be used for multiple multicast group.


sanjoy2006 Wed, 11/28/2007 - 13:52
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Can a single souce transmit max 32 multiple group of receiver

Because one NIC can communicate 32 max multiple group

Am I right pls clear

Harold Ritter Wed, 11/28/2007 - 15:02
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I am not sure I understand your question. What is that 32 multicast group per NIC card you are referring to?


sanjoy2006 Wed, 11/28/2007 - 19:57
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32 different multicast group IDs map to the same Ethernet address

swaroop.potdar Wed, 11/28/2007 - 21:12
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I think you got it mixed up between the Multicast IP to Mapped Mac address limits.

As per the available MAC addresses for multicast switching, only 23 bits are left in the 48 bit mac address to map the multicast iP addresses into the mac addresses.

When a host sends a igmp membership report to join a group, the multicast group is noted for which it wants to join and the hosts physical port number is noted as well. Now the host NIC mac address is not stored in the cam for switching multicast traffic, but the multicast ip is converted to multicast mac address, and as per this conversion, a mcast mac is noted against the port where that host is connected. Hence when any traffic from a source is received from a source for that multicast address it is sent for that mac address at the noted port.

(The above conversion and switching is done in switches if you have enabled IGMP snooping)

Here is a link which explains IGMP snooping with examples which will explain the process.




devang_etcom Thu, 11/29/2007 - 07:59
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Please look at the following link...


Normally what every your requirement is one source should be the source of the multiple Multicast group right!!! so as per the link that solution is providing the 16 streaming of live or on demand MPEG4 data... and as per further discription it is installed on Window operating system... so it seems to be possible... this kind of requirement you can see in IPTV networks...



swaroop.potdar Thu, 11/29/2007 - 12:05
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I am not sure how to start explaining this, but I will give it a try to my understanding.


Look up the MAC Learning section in this link.



Once you have read through then your question would be if switches deliver traffic based on mac address of the end hosts, how is this mapping achieved for multicast addresses, as they are not assigned to any host.

in this case the switch will consider the multicast packet as unknown as will broadcast it. To avoid that a simple mechanism of multicast

IP to mac conversion was introduced which would create these temporary mac's based on the membership report of end hosts

for a certain group. So when this conversion is done apart from the real host mac address pointing to the port where it is connected,

multicast mac also points to the same port. Based on which the switch is able to forward the traffic intelligently to a set of hosts

who want that group received.


Now see this link which explains how Mcast IP to MAC convesion is done.

IP to MAC Conversion


To summarize this, since only 23 bits are available for converting the IP to MAC, the 24th bit is lost .

which in terms of binary to decimal means you have lost 128 from the decimal. so it loops from the start of the multicast range of 224-239 at every 128 bits.

if the starting decimal was the received membership report was for then next would be, , and so on till 239.......

hence when it loops through all the 224 - 239 addres range and it catches up 32 mapped addresses.

Now to conclude,

"32 IP multicast addresses corresponding to each MAC " In here the MAC referenced is not the host mac but the converted MAC.

Even though the conversion is done, because of the available bits the MAC address hence created overlaps 32 multicast IP

addresses. (closest resemblence would be the ACL example with the mask assigned. based on this mask and the starting value

it will keep catching values based on the mask. )

You can try this practically. Assume you have a host and it sends a membership report for Now the switch has to convert this to Mcast MAC.

To do this yourself, write down the binary of this address, and look at the rightmost 23 bits (that is from the left ignore the first 9 binary values.) now if you look at the remaining binary values, these will map and look alike for and so on. And hence if you run through from 224-239 you will get 32 adresses.




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