maybe it will be smart for ftp clients to automatically switch to use PASV when the local machine IP falls into the following range
Address Allocation for Private Internets
10.0.0.0 - 10.255.255.255 (10/8 prefix)
172.16.0.0 - 172.31.255.255 (172.16/12 prefix)
192.168.0.0 - 192.168.255.255 (192.168/16 prefix)
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