This document provides an introduction to data communications and networks. It discusses the components of a data communication system including data flow. It defines what a network is and describes various network criteria such as performance, reliability, and security. It also outlines different physical network structures including point-to-point and multipoint connections as well as topologies like star, bus, ring and hybrid. Furthermore, it categorizes networks into local area networks (LANs), wide area networks (WANs) and metropolitan area networks (MANs) and provides examples of each. It also gives an overview of the Internet and its hierarchical organization. Finally, it defines what a protocol is and describes key protocol elements like syntax, semantics and timing.