I got the Error Message:
Error occurs in downloading files from ftp Server : ftp://speedtest.tele2.net: invalid IPv6 address
String serverAddress = "ftp://speedtest.tele2.net";
FTPClient ftp = new FTPClient();
java.net.preferIPv4Stack -> true
java.net.preferIPv6Addresses -> true
The site has both IPv6 and IPv4 addresses
$ dig AAAA speedtest.tele2.net -> 2a00:800:1010::1 $ dig A speedtest.tele2.net -> 188.8.131.52
IPv6 is usually preferred if available. But while the server can be reached at the IPv4 address port 21 it cannot be reached at the IPv6 address, i.e. a inconsistent setup of FTP and DNS.
The error message from Java is confusing since it is actually not the IPv6 address which is bad but it fails to connect to this address.