The Network Page (Preferences Dialog)

This box includes options for the the active mode of FTP protocol.

To use the active mode, WinSCP needs to know an IP address, via which the server can reach your machine. If you have a direct connection to the server, WinSCP can ask operating system for the IP address. That is the default behaviour, which corresponds to the Retrieve the external IP address from operating system.

But if you are behind NAT router, the operation system would provide only the local network address. Then you need to check Use the following external IP address and fill your external IP address in the box below. You can find your external IP address using many online services, like whatsmyip.net or Google.

When actively listening for incoming connections from the FTP server, WinSCP by default uses a random port number automatically assigned by the operating system. The range of port numbers used by the system is usually 49152 – 65535. If you want to change the range, check the Limit listening ports to checkbox. You will typically want to do that to match your firewall configuration with WinSCP.

Use When SFTP connection is rejected, knock FTP port to make WinSCP automatically try to detect, if server supports FTP protocol,1 when SFTP connection fails. This is to help novice users, who do not realize that WinSCP defaults to the SFTP (contrary to most other file transfer clients).

Read more about Preferences dialog and its other pages.