Publisher Summary This chapter reviews the fundamental concepts that are important in networking. The chapter begins with a review of the Open System Interconnection (OSI) model. The OSI model is used as a reference for all networking protocols. The chapter then describes secure enterprise, emphasizing that the most important thing to remember is that there is no single solution to secure everything. Each company has different restrictions on resources and has different security requirements. This chapter also reviews the basics of firewalls. Firewalls police the network traffic. A firewall is a specialized device that allows or denies traffic based upon administratively defined policies. They contain technologies to inspect network's traffic. The evolution of firewalls has been a long and harrowed path. As new threats come to light, there will always be new technologies that will be created to stop these threats. Segmenting the important hosts is one of the critical things that can be done to secure a network. Knowing networking allows understanding the risks that can occur in a network. When using networking, the more that is known about it, more are the options available.