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I can see MC address assignments HERE. However, I'm looking for what groups in the 225.0.0.x range represent. I know that the MACs for this range overlap the 224.0.0.x range in some (if not all) cases, so maybe that's why there's no table explaining what 225.0.0.1 is (and perhaps why they're 'reserved'). That being said, why would applications ever use groups in the 225.0.0.x range and not the 224.0.0.x range if they know it's going to get pushed to those overlapped ranges regardless? I have 225.0.0.1-3 sitting on some switches now which is why I was curious.

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Technically, no application should be using anything in the 225.0.0.0/24 range. That range is reserved for future use. In most cases, the programmer should actually be using something in the Administratively Scoped Block of 239.0.0.0/8. There are programmers (see multicast questions on Stack Overflow) where the programmers just don't care or actually understand enough about multicast to do it correctly. In fact, I see a lot of questions about network programming in general where the programmer has no business writing network applications because he simply doesn't understand the networking aspects well enough.

Using something in the 224.0.0.0/24 range would limit multicast to the link; the multicast could not be routed if necessary.

As far as overlapping layer-2 multicast addresses go, a layer-2 multicast address represents 32 different layer-3 IPv4 multicast addresses. That is because the layer-3 IPv4 multicast address range has 28 bits for the individual multicast groups, but the 48-bit layer-2 MAC multicast address range only has 23 bits available for the individual multicast groups. The 28 bits for IPv4 addresses minus the 23 bits for MAC addresses is five bits, and 2^5=32.

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  • Ok, so I need to track down the owners of the sources for those groups and ask them what's up... Regarding the overlap topic - Everything destined for example to 225.0.0.1 is also being replicated to anyone in the 224.0.0.1 group and vice versa correct?
    – A L
    Aug 30 '17 at 18:21
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    With IGMP snooping, the frames will be sent to any switch interfaces that have devices subscribed to any of the matching 32 groups. Those devices will look at the layer-3 addresses and reject anything not for the layer-3 multicast groups to which they have subscribed.
    – Ron Maupin
    Aug 30 '17 at 18:30
  • Yep, 100% tracking with you on that. So - as long as those overlapping groups aren't showing up in any of my devices, it won't matter. Thanks again Ron, marking as an answer!
    – A L
    Aug 30 '17 at 18:39

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