Network Security encompasses the policies, practices, and technologies designed to protect computer networks and their data from unauthorized access, misuse, modification, or denial of service attacks—operating across multiple layers from physical infrastructure through application level using defense-in-depth strategies. This discipline has evolved dramatically since early perimeter-based defenses to embrace Zero Trust principles, assuming breach and verifying every access attempt regardless of location, while modern threats like ransomware and AI-powered attacks demand continuous monitoring, behavioral analysis, and automated response capabilities. Understanding network security means recognizing that traditional castle-and-moat approaches are obsolete—attackers already inside networks move laterally for months undetected, making east-west traffic monitoring and micro-segmentation as critical as north-south perimeter controls for containing breaches before they become catastrophic.