Correction: There is unicast! You are using it right now ^_^!
Multicast is a networking term used to mean a protocol in which one packet of information can be send to multiple users instead of sending x packets to all x users (unicast). Suppose your DJing at home and have 30 people tuning in at 64kbps, that's something like 2MB/sec upload on your side, which is not realistic. But while using multicast, this isn't a problem. You just need to send it once. Unfortunately, I haven't seen much development or business incentive with multicast and thats why it's a bit slow to be adopted by companies.
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