During troubleshooting, which layer is responsible for routing, ACLs, MTU, and DNS?

• A. L1
• B. L2
• C. L3
• D. L4

Answer: (C)

The main idea here is that the network layer is responsible for how data moves between networks and what governs access as it travels. Routing decisions are made at this layer to determine the path packets take across multiple networks. Access-control lists are typically configured on routers to permit or deny traffic as it traverses network boundaries, which is a network-layer function. MTU, or the maximum transmission unit, affects what size packets can safely cross a path and how fragmentation is handled, a process that is defined at the network layer and along the IP path. DNS is an application-layer service, but in troubleshooting, its success depends on the network-layer ability to reach the DNS servers and deliver the query to the correct IP address; therefore, DNS issues are often considered alongside network-layer concerns because they impact reachability and routing indirectly. In contrast, physical layer focuses on cables and signals, data-link layer handles MAC addressing and switching, and transport layer deals with end-to-end connections and ports, which do not govern routing or path MTU. So, the layer that best matches routing, ACLs, MTU, and DNS in this troubleshooting context is the network layer.