Which network appliance is used to provide fault tolerance by re-directing traffic when one server in a group becomes inoperable?

The function of a load balancer is essential in network architecture, particularly for providing fault tolerance. When one server within a group becomes inoperable, a load balancer detects this failure and automatically redirects incoming traffic to the remaining operational servers. This process ensures that user requests are always handled promptly, maintaining service availability and performance.

Load balancers can distribute traffic based on various algorithms, such as round-robin or least connections, and can also perform health checks to monitor server performance. By maintaining the continuous availability of applications and services, load balancers play a crucial role in enhancing the reliability of a system and minimizing downtime for users.

In contrast, the other options serve different purposes. A NAT gateway primarily handles network address translation, enabling multiple devices on a local network to access an external network using a single public IP address, but it does not manage server availability. Firewalls are focused on security, controlling incoming and outgoing network traffic based on predetermined security rules, rather than ensuring fault tolerance. Routers direct traffic between different networks but do not provide the fault tolerance mechanisms that load balancers do. Thus, the load balancer is the appliance specifically designed to ensure uninterrupted service by managing and redistributing traffic in the event of server failures.