FTP PORT Mode and PASV Mode in Windows


FTP supports two modes. These modes are called Standard (or Active) and Passive
(or "PASV"). The Standard mode FTP client sends PORT commands to the FTP server.
The Passive mode client sends PASV commands to the FTP Server. These commands
are sent over the FTP command channel.

Standard mode FTP clients first
establish a connection to TCP port 21 on the FTP server. This connection
establishes the FTP command channel. The client sends a PORT command over the
FTP command channel when the FTP client needs to receive data, such as a folder
list or file. The PORT command contains information about on which port the FTP
client receives the data. In PORT Mode, the FTP server always sends data from
TCP port 20. The FTP server must open a new connection to the client when it
sends data.

Passive mode FTP clients also start by establishing a
connection to TCP port 21 on the FTP server to create the control channel. When
the client sends a PASV command over the command channel, the FTP server opens
an ephemeral port (between 1024 and 5000) and informs the FTP client to request
data transfer from that port. The FTP server responds to the request by using
the ephemeral port as the source port for data transfer. When this occurs, the
FTP server does not need to establish a new inbound connection to the FTP
client.

You may need to change the mode that is used by the FTP client,
depending on the firewall configuration on either the FTP client or the server.
Microsoft Internet Explorer 5 and later support both Standard mode and Passive
mode.

How to Change the Internet Explorer FTP Client Mode
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To change the
Internet Explorer FTP client mode:
1. Start Internet Explorer.
2. Click
Internet Options on the Tools menu.
3. Click the Advanced tab.
4. Click
Enable Folder View for FTP sites.
Internet Explorer is an Active mode FTP
client when you select the Enable Folder View for FTP sites option (no matter
what the Use Passive FTP Option says).

Internet Explorer can be enabled
for a passive mode FTP client when you clear this option.