Port 30021
In live entertainment systems, port 30021 is used by MA-Net.
udp/30021
MA-Net — MA-Net3 extended session communication (session 1)
MA Lighting's proprietary session and device-networking protocol linking grandMA2/grandMA3 consoles, onPC stations, NPUs and MA nodes over UDP multicast (plus TCP for large transfers). It is what actually carries console session data, backup/redundancy state and DMX output between everything on the MA network switch, not a separately branded product.
Check it from the command line
npx showstack port 30021
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If you know something else uses port 30021, tell us. Adding it is one file.