just a '-' symbol to let the library use your system's default IP
address. Default FTP operations are passive, and thus won't use PORT.
-Starting in 7.19.5, you can append \&":[start]-[end]\&" to the right of the
-address, to tell libcurl what TCP port range to use. That means you specify a
-port range, from a lower to a higher number. A single number works as well,
-but do note that it increases the risk of failure since the port may not be
-available.
+The address can be followed by a ':' to specify a port, optionally followed by
+a '-' to specify a port range. If the port specified is 0, the operating
+system will pick a free port. If a range is provided and all ports in the
+range are not available, libcurl will report CURLE_FTP_PORT_FAILED for the
+handle. Invalid port/range settings are ignored. IPv6 addresses followed by
+a port or portrange have to be in brackets. IPv6 addresses without port/range
+specifier can be in brackets. (added in 7.19.5)
+
+Examples with specified ports:
+
+.nf
+ eth0:0
+ 192.168.1.2:32000-33000
+ curl.se:32123
+ [::1]:1234-4567
+.fi
You disable PORT again and go back to using the passive version by setting
this option to NULL.