At which OSI reference model layer is routing implemented, enabling logical path selection and connectivity between two end systems?

Introduction / Context:
The OSI reference model divides network communication into seven logical layers, each responsible for specific functions. Routing and logical path selection are key features in internetworking, allowing data to move from a source network to a destination network across multiple intermediate devices. This question checks your understanding of which OSI layer is responsible for routing functions and logical addressing.

Given Data / Assumptions:

We are working with the seven layer OSI reference model.
The focus is on routing and path selection between end systems across networks.
We need to identify the layer that uses logical addressing and routing protocols.

Concept / Approach:
Routing involves forwarding packets between different networks based on logical addresses such as IP addresses. This function is handled at the Network layer, which is Layer 3 in the OSI model. The Network layer provides logical addressing, routing, and path determination. Routers operate at this layer, using protocols such as IP and routing algorithms to decide the best path. The Physical and Data Link layers focus on local transmission of bits and frames, and the Transport layer focuses on end-to-end transport services, not on the path across multiple networks.

Step-by-Step Solution:
Step 1: Recall the responsibilities of each OSI layer, especially the Network layer.Step 2: Associate routing and logical addressing with the layer that uses IP addressing.Step 3: Identify that routers operate at Layer 3, which is the Network layer.Step 4: Select the option that explicitly names the Network layer as handling routing and path selection.

Verification / Alternative check:
As a quick check, remember common devices and their OSI layers. Hubs operate at the Physical layer, switches primarily at the Data Link layer, routers at the Network layer, and some firewalls and gateways may work at higher layers. Since routing is typically done by routers and they are Layer 3 devices, the Network layer must be the correct answer. This reinforces the conceptual mapping between routing and the Network layer.

Why Other Options Are Wrong:
Option A, the Physical layer, is wrong because it deals with transmission of raw bits over a medium and defines electrical and mechanical characteristics, not routing decisions.
Option B, the Data Link layer, is wrong because it is responsible for framing, MAC addressing, and local delivery within a single network segment, not path selection across networks.
Option D, the Transport layer, is wrong because it provides end to end reliability, segmentation, and flow control, but not the routing of packets between different networks.

Common Pitfalls:
Students sometimes confuse the Data Link and Network layers because both involve addressing. Data Link uses MAC addresses for local delivery, whereas the Network layer uses logical addresses like IP for global routing. Another pitfall is to assume that any forwarding is routing, when in fact switching at Layer 2 is different from routing at Layer 3. Keeping the OSI responsibilities and devices clearly mapped helps avoid these errors.

Final Answer:
Network layer (Layer 3)