Book excerpt: This document describes the design concept of the Network Design Tool. The Network Design Tool (NDT) is a collection of analytical techniques, algorithms and simulation methods that may be used to characterize the performance of a computer communication network. Much work has been done over the past several years in network performance analysis and many techniques have been developed or proposed. Each of these methods applies to a particular aspect of the network design and is based on a particular modeling point of view. We define the computer communication network and then describe the different ways the network may be modeled. Each network model is related to the particular design problem being addressed. The various analytical approaches are briefly described and their relationship to the network models discussed. Chapter 2 is a survey of the major approaches to specific network design problems while Chapters 3 and 4 discuss two fairly well defined areas of network analysis: topological design/optimization and protocol validation. Chapter 5 is a survey of network design tools presently available locally or on the Advanced Research Projects Agency Network (ARPANET). Finally, Chapter 6 presents an outline of the NDT specification.