In the OSI network architecture, which layer is responsible for routing packets between different networks?

Introduction / Context:
The Open Systems Interconnection model divides network communication into seven logical layers, from the physical transmission of bits up to application level protocols. Each layer has specific responsibilities. Routing, which involves selecting paths between different networks and forwarding packets across those paths, is associated with a particular OSI layer. This question asks you to identify which layer performs routing functions in the OSI model.

Given Data / Assumptions:

• The OSI layers from bottom to top are Physical, Data link, Network, Transport, Session, Presentation, and Application.
• Routers operate at a specific OSI layer to forward packets between networks.
• Routing decisions are based on logical addresses such as Internet Protocol addresses.
• The options list several layers that handle different aspects of communication.

Concept / Approach:
The network layer, which is layer 3 in the OSI model, is responsible for logical addressing, routing, and path determination. Protocols like Internet Protocol operate at this layer. Routers inspect destination network addresses in the header and decide where to forward the packet. The data link layer, in contrast, deals with physical addressing on a single local network, while the transport layer manages end to end reliability and segmentation. The session layer coordinates communication sessions. Therefore, the correct answer must be the network layer.

Step-by-Step Solution:
Step 1: Recall that routing involves choosing a path between multiple networks, not just sending frames on a local link. Step 2: Recognise that the Internet Protocol operates at the network layer and is used by routers to forward packets. Step 3: Identify that the transport layer uses ports and manages end to end connections but does not perform routing. Step 4: Note that the session layer handles dialogue control and the data link layer handles frames and physical addressing on a single segment. Step 5: Select the network layer as the layer that performs routing according to the OSI model.

Verification / Alternative check:
If you examine router configuration commands and protocols such as Open Shortest Path First, Routing Information Protocol, or Border Gateway Protocol, they all deal with network layer addressing and metrics. Network engineering diagrams also show routers operating between different local area networks at layer 3, while switches that work primarily at the data link layer do not perform traditional routing. This practical experience confirms that routing is a network layer function.

Why Other Options Are Wrong:
Option a, the transport layer, is responsible for reliable data transfer, flow control, and segmentation for protocols such as Transmission Control Protocol and User Datagram Protocol, not for routing. Option b, the session layer, organises and synchronises dialogues between applications. Option d, the data link layer, operates on frames within a single network segment and uses Media Access Control addresses; it does not choose paths between networks.

Common Pitfalls:
Students sometimes confuse switching with routing and believe that any device that forwards traffic performs routing. However, traditional layer 2 switches operate at the data link layer, forwarding frames based on Media Access Control addresses, while routers operate at the network layer using logical addresses. Remembering the OSI layering and associating routing explicitly with layer 3 helps avoid this confusion.

Final Answer:
In the OSI model, the network layer is responsible for routing packets between different networks.