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Is it possible to use multicast on the public internet? If yes: How? Are special IP addresses required and where do you get them from?

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You cannot multicast on the public Internet, but you can multicast across the public Internet to another site by using a tunnel that supports multicast.

Multicast routing is very different from unicast routing, and all the routers in the path of the multicast packets need to have multicast routing configured.

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    @Darren IPTV typically uses many unicast connections. – Ole Tange Feb 12 '18 at 14:55
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    @Darren, don't confuse multicasting IPTV on a company network with multicasting on the public Internet. Some ISPs may even have multicast set up in their own networks for their customers, but multicasting is not enabled across the public Internet. – Ron Maupin Feb 12 '18 at 15:02
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    @Darren, your ISP's network is not the Internet. The Internet is the connections between the ISPs. – Ron Maupin Feb 12 '18 at 15:10
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    @Darren, many people believe the Earth is flat, but that doesn't make it so. This is Network Engineering, where we deal with "professionally managed networks in a business environment," and home networking questions are explicitly off-topic. Professional network engineers should know what the public Internet is. – Ron Maupin Feb 12 '18 at 15:18
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    The basic point is that multicast routing is not supported across the entire Internet. Any specific network (either a local network or an ISP backbone) might support multicast for their users, but you can't do multicast outside that scope. – Barmar Feb 12 '18 at 19:29
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As an end-user, you cannot multicast across the Internet, unless using a tunnel.

As a larger organization, like a video provider or an ISP, it is certainly possible to forward multicast packets across their domain boundary (i.e. across an Internet).

How ?

Essentially, to forward multicast packets within your own domain (or Autonomous System, AS), you use the PIM protocol and multicast routing. To forward those multicast packets to another AS (like another ISP), you would need a peering agreement with them and use the Multicast Source Discovery Protocol (MSDP), configured on both ends.

While you won't propagate your multicast across the global Internet, crossing network boundaries with multicast packets is not impossible.

PIM and MSDP are advanced, CCIE-level concepts. Here is a good Cisco white paper about it : https://www.cisco.com/c/en/us/td/docs/ios/solutions_docs/ip_multicast/White_papers/mcst_ovr.html#wp1015335

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