Which layer of the OSI model deals with routing between networks?

A. Data Link
B. Network
C. Transport
D. Session

Answer: B. Network
Explanation:

OSI Model Layer Breakdown

•  Network Layer (Layer 3): This layer handles routing and logical addressing (IP addresses).  It decides which physical path the data should take based on network conditions, priority, and other factors.  

  •  Data Unit: Packets.  

  •  Key Devices: Routers and Layer 3 switches.  

• Data Link Layer (Layer 2): This layer is responsible for node-to-node data transfer. It deals with physical addressing (MAC addresses) and error detection for the local link.  

  • Data Unit: Frames.

  • Key Devices: Switches and Bridges.

•  Transport Layer (Layer 4): This layer manages end-to-end communication, ensuring data is delivered reliably and in the correct sequence.  It handles flow control and error correction. 

  •  Protocols: TCP and UDP.  

  •  Data Unit: Segments (TCP) or Datagrams (UDP).  

•  Session Layer (Layer 5): This layer establishes, manages, and terminates connections (sessions) between local and remote applications.  

Why the other options are incorrect

• A. Data Link: While it handles communication, it is limited to the local network segment.  It cannot "route" traffic between different networks; it only moves data between adjacent devices.

• C. Transport: This layer is concerned with how the data is sent (reliably vs. quickly) once the path has already been established by the Network layer.  

• D. Session: This is a much higher-layer function that deals with the "dialogue" between two software applications, not the physical or logical routing of data through wires and routers.  

The "CISSP Rule of Thumb": If you see IP Addresses or Routing, think Layer 3 (Network). If you see MAC Addresses or Switches, think Layer 2 (Data Link).