Again, when you are using a router, it creates an internal network.
Each computer on the network has a unique IP number. The first two
numbers, "192.168..." are designated for an internal network, and
cannot be accessed by anyone on the Internet. Remember, when you
run an FTP server, you need to give your users your outside IP
address, (WAN) not your internal IP address, (LAN).

Doing this is not recommended. Depending on your configuration,
it might work or it might not work. If PASV mode is not working, then
this might very well be the cause. Even if PASV mode is working after
redirecting PASV mode to an internal IP, that still does not
mean that you should do that.

If you are using a newer version of Serv-U FTP Server and if it redirecting
PASV mode to an internal (LAN) IP address, then that means that you
neglected to use steps 2 and 3 in the PASV mode section of
Serv-U FTP Server.

This mistake will produce several problems. You will see this error in your log file:
"<- Unknown list format."

Also, when you get into the FTP server, you will not see any folders.
Instead you will only see "220" or something similar.

This mistake will also make your server freeze when your FTP user tries
to use PASV mode.

To solve this problem, make sure that the FTP server port and the PASV
port range are seperate.

When this mistake is made, the PASV mode tries to connect at the port that
the FTP server is running on, so the server responds with the welcome message
(220) because it thinks that someone is logging onto the server.

If your router/firewall cannot forward enough ports to avoid this problem
then many people will have problems using your server.

Doing this is not recommended either. Depending on your configuration,
it might work or it might not work. If PASV mode is not working, then
this might also be the cause. It is recommended that you redirect PASV
mode to the port range of 50000 to 50100. Even if PASV mode is working
after redirecting PASV mode to a port range other than 50000 to 50100,
that still does not mean that you should do that either.

This problem is usually caused by the DHCP server on a router when Static IP addresses
are not defined for your LAN under Windows.

You need to "forward" or "route" all Internet traffic that the FTP
server uses to the computer running the FTP server on a port. That is
what a router does. Also, you should disable all software firewalls
(such as the one which comes with Windows XP) or you should tell your
software firewall to allow you to open the ports that your FTP server
is using.

Several things can cause this problem. Most of the time, it's
because they should be using PASV mode instead of PORT mode. The
person who is trying to access your FTP server should try both PASV
and PORT modes to see which works best.

This problem can be caused by your ISP (Internet Service Provider)
changing your outside (WAN) IP address. When your outside
(WAN) IP address changes, that means that your IP is dynamic. You
need to inform your users of your new IP address. Another way to
fix this problem is to use a dynamic DNS service provider
such as no-ip.com. Sign up for an account with
them and keep the software that they give you updated.
This problem can also be caused by a software firewall.

Some Internet Service Providers block incoming traffic on port 21.
Try running your FTP server on a different port other than port 21
if you wish to verify that your Internet Service Provider is blocking
incoming traffic on port 21.Don't forget to forward the new port on your
router.

If your ISP is blocking a port, then on the GRC test, it will show up as stealth.

You should tell your FTP server to limit the amount of speed that
your FTP server is using.

Also, if you have family or friends that are using other computers in
your local area network, they might be downloading and/or uploading
stuff, and reducing your Internet speed.

There are unscrupulous users on your FTP site who are making these
directories, who are trying to hide them from you and have access to
whatever they put into that directory for themselves. To delete them,
you have to have some expertise in DOS.

Some people are testing your FTP site to see how fast it is. The
files are usually a specific size so that they can calculate the
speed.

If you are running IIS (Internet Information Services) then another
FTP server is using port 21. You have to go to "add/remove programs
windows components" to stop the FTP server service from running.

If you are getting a:

"425 Cannot open data connection (10061)."

Or, if you are getting any error under PORT mode: (100xx)

Or, an "Error 530 Port command only accepts Client IP address"
error in your log when your FTP user is using PORT mode, then it
is usually the result of the person connecting to your FTP also being
behind a firewall/router. One sure way to fix this problem is using
PASV mode. If your FTP user has a spare 56k Internet connection that
he/she can use, try PORT mode again on the 56k connection.
Then it should work.

The error, ""Error 530 Port command only accepts Client IP address"
appears because the person connecting to your FTP is also being
behind a firewall/router. The "Client IP address" it talks about should be
their OUTSIDE IP address.

You need to get your PASV mode working by specifying a "PASV port
range" for your FTP server to use. You then need to forward that port
range on your router to the computer that is running the FTP server.

Many programs often use the port range of 1024 - 4999.

Try running your FTP server on a port that is not in that range.