Computers and information networks are critical to the success of businesses, both large and small. They connect people, support applications and services, and provide access to the resources that keep the businesses running. To meet the daily requirements of businesses, networks themselves are becoming quite complex.

Network Requirements
Today, the Internet-based economy often demands around-the-clock customer service. This means that business networks must be available nearly 100 percent of the time. They must be smart enough to automatically protect against unexpected security incidents. These business networks must also be able to adjust to changing traffic loads to maintain consistent application response times. It is no longer practical to construct networks by connecting many standalone components without careful planning and design.

Building a Good Network
Good networks do not happen by accident. They are the result of hard work by network designers and technicians, who identify network requirements and select the best solutions to meet the needs of a business.

The steps required to design a good network are as follows:
Step 1. Verify the business goals and technical requirements.
Step 2. Determine the features and functions required to meet the needs identified in Step 1.
Step 3. Perform a network-readiness assessment.
Step 4. Create a solution and site acceptance test plan.
Step 5. Create a project plan.

Network Requirements
Most businesses actually have only a few requirements for their network:

• The network should stay up all the time, even in the event of failed links, equipment failure, and overloaded conditions.
• The network should reliably deliver applications and provide reasonable response times from any host to any host.
• The network should be secure. It should protect the data that is transmitted over it and data stored on the devices that connect to it.
• The network should be easy to modify to adapt to network growth and general business changes.
• Because failures occasionally occur, troubleshooting should be easy. Finding and fixing a problem should not be too time-consuming.

Fundamental Design Goals
When examined carefully, these requirements translate into four fundamental network design goals:

• Scalability: Scalable network designs can grow to include new user groups and remote sites and can support new applications without impacting the level of service delivered to existing users.
• Availability: A network designed for availability is one that delivers consistent, reliable performance, 24 hours a day, 7 days a week. In addition, the failure of a single link or piece of equipment should not significantly impact network performance.
• Security: Security is a feature that must be designed into the network, not added on after the network is complete. Planning the location of security devices, filters, and firewall features is critical to safeguarding network resources.
• Manageability: No matter how good the initial network design is, the available network staff must be able to manage and support the network. A network that is too complex or difficult to maintain cannot function effectively and efficiently.

Hierarchical Network Design
In networking, a hierarchical design is used to group devices into multiple networks. The networks are organized in a layered approach. The hierarchical design model has three basic layers:

• Core layer: Connects distribution layer devices.
• Distribution layer: Interconnects the smaller local networks.
• Access layer: Provides connectivity for network hosts and end devices.

Hierarchical networks have advantages over flat network designs. The benefit of dividing a flat network into smaller, more manageable hierarchical blocks is that local traffic remains local. Only traffic destined for other networks is moved to a higher layer.

Layer 2 devices in a flat network provide little opportunity to control broadcasts or to filter undesirable traffic. As more devices and applications are added to a flat network, response times degrade until the network becomes unusable. Figures 1-1 and 1-2 show the advantages of a hierarchical network design versus a flat network design.

Network Design Methodologies
Large network design projects are normally divided into three distinct steps:

• Step 1. Identify the network requirements.
• Step 2. Characterize the existing network.
• Step 3. Design the network topology and solutions.

Step 1: Identifying Network Requirements The network designer works closely with the customer to document the goals of the project. Figure 1-5 depicts a meeting between the designer and the business owner. Goals are usually separated into two categories: n Business goals: Focus on how the network can make the business more successful n Technical requirements: Focus on how the technology is implemented within the network.

Step 2: Characterizing the Existing Network Information about the current network and services is gathered and analyzed. It is necessary to compare the functionality of the existing network with the defined goals of the new project. The designer determines whether any existing equipment, infrastructure, and protocols can be reused, and what new equipment and protocols are needed to complete the design.

Step 3: Designing the Network Topology A common strategy for network design is to take a top-down approach. In this approach, the network applications and service requirements are identified, and then the network is designed to support them.

When the design is complete, a prototype or proof-of-concept test is performed. This approach ensures that the new design functions as expected before it is implemented.