This first chapter has set the stage for the book; as you can see, you must understand defense in depth to improve the security of a networked organization. No silver bullets exist, and no single component can properly defend a network. You can deploy many components working together in such a way as to make attack difficult. Defense in depth describes the process of layering these components to capitalize on their respective strengths. It is flexible, but no single roadmap can select and deploy the various perimeter components. Our role is to design, build, and maintain the perimeter so that the overall security of the network is at an acceptable level, while providing an environment that supports business operations of the organization. A defense-in-depth approach can be used to secure an individual machine or the largest network in the world. It is a powerful tool for defenders.