|  | 3.3 | How to Identify and Select Networking Capabilities
| 3.3.1 | The use of the OSI model in WAN design
After you understand your networking requirements, you must identify and then design the computing environment to meet these requirements. The following sections will help you with these tasks. Hierarchical models for network design allow you to design networks in layers. To understand the importance of layering, consider the OSI model, a layered model for understanding computer communications. By using layers, the OSI reference model simplifies the tasks required for two computers to communicate. Hierarchical models for network design also use layers to simplify the tasks required for internetworking. Each layer can be focused on specific functions, thereby allowing the networking designer to choose the right systems and features for the layer. Using a hierarchical design can facilitate changes. Modularity in network design allows you to create design elements that can be replicated as the network grows. Also, because networks will require upgrades, the cost and complexity of making the upgrade are constrained to a small subset of the overall network. In large flat or meshed network architectures, changes tend to affect a large number of systems. You can also facilitate the identification of failure-points in a network by structuring the network into small, easy-to-understand elements. Network managers can easily understand the transition points in the network, which helps identify failure points.