What networking solution allows external users access only to an internal FTP server?

The correct answer is port forwarding. This networking solution enables external users to access specific services within a private network, such as an FTP server, by directing incoming traffic from a designated external port to the internal IP address of the server.

In a typical scenario, a network might have a firewall that restricts access to internal resources. By configuring port forwarding, the firewall or router can be set up to listen for incoming connections on a specific port (for example, port 21 for FTP) and then forward those requests to the internal IP address of the FTP server. This method allows external users to connect to the FTP server, while keeping other internal network resources secure and inaccessible.

Other options, while related to networking, do not specifically provide the functionality needed for controlled external access to just the FTP server. A VPN configuration, for instance, would grant broader access to the internal network rather than limiting access to a single service. Routing tables are primarily used for directing traffic within networks rather than managing external access to internal services. Dynamically assigned IP addresses pertain to how devices obtain IP addresses rather than providing a means of controlled external access to internal resources.