The network layer (layer 3) defines how packets are routed and relayed between end systems on the same network or on interconnected networks. Message routing, error detection and control of node traffic are managed at this level.
Reference(s) used for this question: KRUTZ, Ronald L. & VINES, Russel D., The CISSP Prep Guide: Mastering the Ten Domains of Computer Security, John Wiley & Sons, 2001, Chapter 3: Telecommunications and Network Security (page 82).