Unassigned by IANA; in practice the ADB socket for the first Cuttlefish virtual Android device.
IANA leaves 6516-6542 unassigned. The concrete use of 6520 is Android's Cuttlefish virtual device platform: each instance exposes an ADB connection on a TCP server socket, 6520 for instance 1, 6521 for the second, and upward, so adb connect 127.0.0.1:6520 reaches the first VM. You meet it on Android build and CTS machines running emulated devices. ADB over TCP has no authentication once a host is authorised, which is why Cuttlefish binds it locally.
Do not expose
A reachable ADB endpoint is a root-capable shell on the device plus arbitrary file push and pull; keep it on loopback and reach it over an SSH tunnel.
$ adb connect 127.0.0.1:6520Seen 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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