Telecommunication networks based on optical fiber technology have become a major information transmission system with high capacity optical fiber links encircling the globe in both terrestrial and undersea installation. At present there are numerous passive and active optical devices within a light wave link that perform complex networking functions in the optical domain, such as signal restoration, routing, and switching. Along with the need to understand the functions of these devices comes the necessity to measure both components and network performance and to model and simulate the complex behavior of reliable high capacity networks. This chapter presents the fundamental principles for understanding and applying these issues. This chapter is primarily about TCP/IP network protocols and Ethernet network architectures, but also briefly describes other protocol suites, network architectures, and other significant areas of networking. It explains in simple terms the way networks are put together, and how data packages are sent between networks and subnets, along with how data is routed to the Internet.