http://*.com/questions/14388706/socket-options-so-reuseaddr-and-so-reuseport-how-do-they-differ-do-they-mean-t
SO_REUSEADDR socketA socketB Result
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ON/OFF 192.168.0.1:21 192.168.0.1:21 Error (EADDRINUSE)
ON/OFF 192.168.0.1:21 10.0.0.1:21 OK
ON/OFF 10.0.0.1:21 192.168.0.1:21 OK
OFF 0.0.0.0:21 192.168.1.0:21 Error (EADDRINUSE)
OFF 192.168.1.0:21 0.0.0.0:21 Error (EADDRINUSE)
ON 0.0.0.0:21 192.168.1.0:21 OK
ON 192.168.1.0:21 0.0.0.0:21 OK
ON/OFF 0.0.0.0:21 0.0.0.0:21 Error (EADDRINUSE)