Questions about broadcasts & multicasts - TCP-IP

This is a discussion on Questions about broadcasts & multicasts - TCP-IP ; I'm trying to understand better why programs sometimes multicast to 224.0.0.2.... sleezy programs that should never, ever, require any network access. It would be very easy to just set the firewall to block it and forget it, but I am ...

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Thread: Questions about broadcasts & multicasts

  1. Questions about broadcasts & multicasts

    I'm trying to understand better why programs sometimes multicast to
    224.0.0.2.... sleezy programs that should never, ever, require any
    network access. It would be very easy to just set the firewall to
    block it and forget it, but I am in the industry and would like to
    understand exactly what is going on.

    Specific Questions:

    1) From what I understand ( http://en.wikipedia.org/wiki/Multicast_address
    ) the multicast is intended for all routers on the subnet. So I assume
    the program is looking for the gateway to the internet?

    2) If it wanted that info couldn't it just tell what Windows default
    gateway is set to and use that? So why waste bandwidth and polute my
    LAN with 224 multicasts??? Again, not that I'm worried about it, I
    just want to understand.

    3) Regarding the 224.0.0.1 multicast address... I notice on the above
    wikipedia page that it is intended for all hosts on the subnet. As is
    the 255.255.255.255 broadcast address. So Im a bit confused about the
    difference between them, they both go to all hosts, can anyone clarify
    without using big words?



    Thanks in advance!






  2. Re: Questions about broadcasts & multicasts

    On Apr 7, 6:06 pm, Dennis wrote:
    > I'm trying to understand better why programs sometimes multicast to
    > 224.0.0.2.... sleezy programs that should never, ever, require any
    > network access. It would be very easy to just set the firewall to
    > block it and forget it, but I am in the industry and would like to
    > understand exactly what is going on.
    >
    > Specific Questions:
    >
    > 1) From what I understand (http://en.wikipedia.org/wiki/Multicast_address
    > ) the multicast is intended for all routers on the subnet. So I assume
    > the program is looking for the gateway to the internet?


    Yes.

    > 2) If it wanted that info couldn't it just tell what Windows default
    > gateway is set to and use that? So why waste bandwidth and polute my
    > LAN with 224 multicasts??? Again, not that I'm worried about it, I
    > just want to understand.


    The default gateway may not support things like UPnP while another
    gateway might.

    > 3) Regarding the 224.0.0.1 multicast address... I notice on the above
    > wikipedia page that it is intended for all hosts on the subnet. As is
    > the 255.255.255.255 broadcast address. So Im a bit confused about the
    > difference between them, they both go to all hosts, can anyone clarify
    > without using big words?


    224.0.0.1 means all hosts in the multicast domain while
    255.255.255.255 means all hosts in the broadcast domain. These may not
    be the same thing.

    DS

  3. Re: Questions about broadcasts & multicasts

    On Apr 7, 9:06*pm, Dennis wrote:

    > 3) Regarding the 224.0.0.1 multicast address... I notice on the above
    > wikipedia page that it is intended for all hosts on the subnet. As is
    > the 255.255.255.255 broadcast address. So Im a bit confused about the
    > difference between them, they both go to all hosts, can anyone clarify
    > without using big words?


    I agree with David's replies, but wanted to add something to this one.

    Turns out, IPv6 does exactly what you ask. IPv6 gets rid of the "all
    1s" broadcast, and replaces it with a multicast to all "on-link
    hosts."

    The difference between the two is that an all 1s broadcast maps
    algorithimically to an all 1s MAC broadcast address. Which means that
    every host in the Ethernet is interrupted. In the days before IP was
    the universal L3 protocol, this meant that all DECnet, IPX, Appletalk,
    SNA, etc. hosts on the L2 network were interrupted by something that
    was irrelevant to them.

    Instead, if a multicast address like 224.0.0.1 is used, it maps to a
    MAC multicast address which only IP hosts would monitor. As things are
    today, perhaps the difference is not so important anymore.

    Bert

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