CC by openclipart.org
For the client in this scenario I'm running a Raspberry Pi 3 Model B which I have plugged into an ethernet cable on my home network. I'm using my Ubuntu 18.04 laptop to serve up the multicast stream.
If you're new to multicast the first piece of reading you should do is to read about the reserved IP ranges on Wikipedia. You can't broadcast on just any old address, but you do have a very wide selection to choose from. I decided to broadcast on udp://126.96.36.199 and port 5000, because why not?
I downloaded a video from https://www.stockio.com/free-videos/ and saved it into a folder. From there all you need to do is run ffmpeg on the server, thusly:
ffmpeg -re -i costa.mp4 -bsf:v h264_mp4toannexb -acodec copy -vcodec copy -f mpegts udp://188.8.131.52:5000
There are actually some parameters in there that ffmpeg gave me in its error output when I tried to play the file. If you use the same file as me then you need to include them, otherwise you might not need to.
Once ffmpeg had started streaming the file I quickly opened up VLC on the Pi and tuned to the network stream that my laptop was streaming on. Hurray! Everything is working, my UDP multicast is up and running, now I can play with some more advanced features.
Here is a screenshot from my Pi: