Which of the following is NOT considered a boundary protection device?

Gateways typically serve as points of entry and exit for data traveling between different networks, thus facilitating communication and potentially enforcing security policies. While they do offer protections like network address translation or intrusion detection, they are generally focused on allowing traffic between distinct networks rather than explicitly preventing unauthorized access or monitoring data flow at a boundary.

In contrast, routers are critical for directing data packets between networks, and they can incorporate features like access control lists to enhance security. Proxies act as intermediaries for requests from clients seeking resources from other servers, allowing for content filtering and anonymization, which are essential aspects of boundary protection. Data diodes are designed to allow data to flow in only one direction, providing a high level of security by preventing unauthorized access from the outside.

Thus, while gateways have some protective qualities, they are not primarily classified as boundary protection devices compared to the other options.