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  1. Hello Rene,
    A few questions and confusions.

    1. When a multicast address is being used as a group address(for example, this address is assigned to the multicast server and this address has to have route in the network design because whenever a host will encapsulate an IP packet, it will use its own address as the source and address as the destination address. Is this correct?
    2. Would you also please give me a real life scenarion where multicast is used?
    3. In your IGMP version 3 example, is being used as the source address. What is this address for? As far as my understanding goes, source IP address is the host IP and destination is the Multicast IP address( Please explain it little bit.
    4. One difference between ver 2 and ver 1 is that in ver 1 all the hosts send report to the router whereas in ver 2 only host sends report to the router on behalf of all the hosts in the same group. Is it correct?

    Thank you so much


  2. Hello Azm

    Well, not quite. The multicast address is not assigned to the multicast server, but it is the destination used for multicast traffic FROM the server TO the members of the group. Secondly, under what circumstances would a host communicate with When it makes a request to join the multicast group. This in essence is a communication between the host and the IGMP functionality of the router, not with the multicast server. This communication would have a source IP of the host and a destination IP of the mutlicast IP, but that destination in such a communication is the router.

    * Videoconferencing
    * IPTV/Digital TV/Cable TV
    * Live video transmission over the Internet
    * Webinars
    * online radio

    Here Rene is indicating that a host can also include a source IP address in the membership report messages and this address can be configured. Because the host is a router, it can state which one of its IP addresses is the source for the specific multicast transaction. Rene has added a fictitious IP address just to see it easily in the debugging and wireshark captures. In a production environment, this IP address should be that address that will be added to the multicast group.[quote=“azmuddincisco, post:6, topic:1123”]
    4) One difference between ver 2 and ver 1 is that in ver 1 all the hosts send report to the router whereas in ver 2 only host sends report to the router on behalf of all the hosts in the same group. Is it correct?

    I’m not sure I understand what you mean. Can you clarify the second part of your question?

    I hope this has been helpful!


  3. Hi Azm,

    Multicast isn’t really used on the Internet. It would make a lot of sense to use multicast for online radio but in reality, it’s unicasted everywhere. There is no “global” multicast network that spans multiple ISPs. One application that was common back in the days on the LAN was Norton Ghost to send a system image to all computers. Without multicast, you had to unicast huge images to all computers which wasn’t very efficient.

    When your computer wants to receive a multicast stream, it uses IGMP to “report” which multicast group it wants to receive. IGMP version 2 is very common but it doesn’t allow you to specify a source which means that everyone can send packets to With IGMP version 3, you can specify the source which means you can report that you only want to receive traffic for multicast group from source x.x.x.x.

    Once a multicast enabled router receives your IGMP packet, it’s up to PIM. It really depends if you are using PIM dense or PIM sparse mode how the multicast traffic is flooded in the network. PIM dense uses a “flood and prune” model while PIM sparse uses a “join” mechanism to request the multicast traffic from an upstream router. These two lessons explain that process:

  4. Hi Rene,

    What is the purpose of sending two membership report from H1, one determines there is a new address and the other determines the mode, I mean why not be in one membership report ???

  5. Hi Hussein,

    To be honest, I have no idea…I guess this is one of those “that’s how they designed it” answers. They probably could have been combined in a single packet but for some reason, they decided to go for two packets.

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