OSI model fundamentals Within the OSI networking model, which layer is responsible for routing packets between different networks?

Introduction / Context:
The OSI model divides networking functions into seven layers, each with specific responsibilities. Routing—finding paths between networks—is central to internetworking and is performed by a specific layer that handles logical addressing and path selection.

Given Data / Assumptions:

• OSI layers: Physical, Data Link, Network, Transport, Session, Presentation, Application.
• Routing entails moving packets across network boundaries using logical addresses.
• We must select the layer responsible for this function.

Concept / Approach:
The Network layer (Layer 3) provides logical addressing (e.g., IP), routing, and path determination. Routers operate at this layer, choosing next hops and applying policies such as metrics and access control lists. Other layers provide reliability, framing, or session control, but not routing across networks.

Step-by-Step Solution:
Identify routing as inter-network path selection.Map routing to Layer 3 functions: addressing, fragmentation, forwarding.Confirm devices: routers work at the Network layer.Select “network layer.”

Verification / Alternative check:
Consider examples: IP routing tables, OSPF, BGP—all are Layer 3 protocols and mechanisms, which validates the answer.

Why Other Options Are Wrong:
Session layer manages dialogues between endpoints; it does not route.Transport layer provides end-to-end reliability and flow control (e.g., TCP), not routing.Data Link layer handles local network framing and MAC addressing within the same broadcast domain.

Common Pitfalls:

• Confusing switching (Layer 2) with routing (Layer 3).
• Assuming TCP ports (Layer 4) are used for routing; they are for multiplexing services.
• Mixing OSI terminology with TCP/IP model layers without mapping carefully.

Final Answer:
network layer