Flask's development server port, the Docker registry's container port, and the port macOS quietly took over.
flask run serves on 127.0.0.1:5000, and the Docker distribution registry listens on 5000 inside its container. On macOS Monterey and later the AirPlay Receiver service binds 5000 by default, which is why Flask developers on Macs see "Address already in use" out of nowhere; the fix is to turn AirPlay Receiver off in System Settings or pick another port. IANA has it registered as commplex-main.
Do not expose
Flask's docs say the dev server is not designed to be secure, and an unauthenticated registry on 5000 lets anyone push and pull images.
$ sudo lsof -nP -iTCP:5000 -sTCP:LISTENSeen this on your network? Beige Box scans your Wi-Fi and shows every open port on every device — and its Ports tool links straight back here.
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Corrections or a missing port? Reply @rimrocksystems.