Which solution allows external users to access only an internal FTP server on a private network?

The solution that allows external users to access only an internal FTP server on a private network is through a method called Port Forwarding. Here are the steps:

1. Identify the FTP server's private IP address: The first step is to identify the private IP address of the FTP server on your private network. This is the address that your router uses to forward FTP traffic to.

2. Access your router's configuration settings: You can usually do this by typing your router's IP address into a web browser. The IP address is typically something like 192.168.1.1 or 192.168.0.1.

3. Find the Port Forwarding section: This will be located in different places depending on your router's manufacturer and model. It might be under a section called "Advanced Settings" or "Firewall".

4. Create a new Port Forwarding rule: You'll need to enter the FTP server's private IP address, the port number for FTP (which is typically 21), and the protocol (which should be set to TCP).

5. Save your changes and test the connection: Once you've saved your new rule, you should test the connection from an external network to make sure everything is working correctly. You can do this by typing your public IP address into an FTP client, followed by the port number (e.g., "123.456.789.012:21").

Remember, port forwarding can expose your network to potential security risks, so it's important to only forward the necessary ports and to regularly monitor your network's traffic.