Which scenario is an example of when a gateway is required?

A gateway functions as a translator or a bridge between different network protocols, allowing communication between different systems that may not share the same data formats or protocols. In the scenario of connecting an IP network to an AppleTalk network, a gateway is essential because these two networks utilize different protocols for data transmission. The gateway will translate the data from one format to another, enabling devices on either network to communicate effectively, despite the inherent differences in their protocol structures.

The other scenarios may involve networking principles, but they do not require a gateway. For instance, connecting a local area network usually involves switches or routers, not a gateway, since these devices can manage communication within the same protocol and domain. Isolating network segments with a bridge focuses on using a layer 2 device that connects and filters traffic between network segments rather than translating protocols. Likewise, implementing a firewall pertains to security and filtering traffic based on rules rather than converting data formats or bridging different protocol networks. Thus, the role of a gateway becomes crucial specifically in scenarios where interoperability between distinct networking protocols is needed.