This document provides an introduction to network security by explaining some foundational networking concepts. It defines what a network is, describes the ISO/OSI reference model for layered network communications, and gives overviews of popular early networks like UUCP and the Internet. It then focuses on TCP/IP, the core protocols that power the Internet. The document also discusses types of network threats like denial-of-service attacks and unauthorized access, as well as defenses like firewalls that create barriers between internal and external networks. It emphasizes that defining security policies is key to building a secure network.