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If I am streaming on a host using the internet outside my nat, how is multcast addressing work with respect to streaming for example, if the application outside sends to 224.0.0.1 how is the packet finding its way to my machine?

  • Besides what @Zac67 explains, your example address, 224.0.0.1 is a link-local multicast, meaning it cannot be sent off the local link, even within your own network. There are different multicast ranges, and anything in the 224.0.0.0/24 range is confined to the local link. – Ron Maupin Jun 18 at 14:35
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First off, 224.0.0.1 is a very bad choice of example multicast address. (Why is left up to the reader to learn.)

If you want multicast traffic to span your firewall -- or a NAT router pretending to be one -- explicit rules will be required. And some level of multicast routing, or IGMP proxy will be needed. In some simple cases, it may suffice to simply bridge multicast, but that can be problematic. (multicast is uni-directional; if the thing(s) listening to a stream ever want to communicate with the sender, it will have to be done via unicast.)

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  • Thanks! that was what I was wondering for some streams that may be real time what type of circuit is being set up how is it getting through certain firewalls. I will figure that out appreciate you pointing me in the right direction. – meanthatmuchtoyou Jun 19 at 0:24
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Generally, multicast doesn't work on the public Internet. You can only use it on a private LAN or your own public subnet. Some tunneling protocols support multicast as well.

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  • so when a video game server is streaming to clients using multicast, it basically sets up its own little subnet among these clients? – meanthatmuchtoyou Jun 18 at 14:40
  • @meanthatmuchtoyou, I do not know any game servers that use multicast. Check on Stack Overflow, they all do use UDP, but it is unicast. – Ron Maupin Jun 18 at 14:52
  • Thanks Ron and Zac explains a lot. A game server didn't end up being a good example sorry lol. – meanthatmuchtoyou Jun 19 at 0:21

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