A router that reads only the network layer header and forwards without altering the transport layer header operates at which OSI layer?

A router that reads only the network layer header and forwards without altering the transport layer header operates at which OSI layer?

• A. Layer 2
• B. Layer 3
• C. Layer 4
• D. Layer 5

Answer: (B)

Routing packets based on IP addresses is a network-layer function. The router uses the network-layer header to decide where to send the packet and typically does not modify the transport-layer header, such as TCP or UDP ports. This behavior aligns with the network layer, Layer 3 of the OSI model. Layer 2 handles data-link aspects like MAC addresses and frames, Layer 4 deals with transport and ports, and Layers 5 and above cover higher-level sessions and applications, which aren’t involved in this forwarding decision. So the device operates at Layer 3.

Routing packets based on IP addresses is a network-layer function. The router uses the network-layer header to decide where to send the packet and typically does not modify the transport-layer header, such as TCP or UDP ports. This behavior aligns with the network layer, Layer 3 of the OSI model. Layer 2 handles data-link aspects like MAC addresses and frames, Layer 4 deals with transport and ports, and Layers 5 and above cover higher-level sessions and applications, which aren’t involved in this forwarding decision. So the device operates at Layer 3.