Network Design Step 3: Defining Network Topologies and Services

After the individual modules of the new or upgraded network are defined, the specific topologies, technologies, protocols, and network services should be designed. Examples of these elements include:

Network topology design. The design of the new network topology needs to consider both the LANs to be implemented in any offices, as well as the WAN links interconnecting locations. Both the physical and logical elements of the proposed network should be documented, with appropriate diagrams for each. For equipment, a physical diagram would show the interconnection of network elements, while a logical diagram would display the layout of elements like VLANs, network modules, and so on.

Network technology design. The technologies associated with the new network also need to be planned at diagrams. For example, the design may specify that Fast Ethernet is to be used for client connection at the access layer, while the links connecting different layers use redundant fiber-optic Gigabit Ethernet in a redundant design. For WAN links, the technologies to be used should be specified, along with information about key elements such as link speeds.

Network protocol design. The network protocols to be implemented on the new network needs to be specified, along with their scope. For example, the entire network may run TCP/IP as its primary protocol, but certain portions of the network may require the use of other protocols such as IPX/SPX or NetBEUI.

Logical addressing design. The network design needs to include information about the design of logical addressing on the network. For example, a network may use public or private IP addresses internally, along with different hierarchical addressing schemes like VLSM or CIDR. In the case of private IP addressing, connectivity through the use of services like network address translation (NAT) or proxy servers must also be addressed.

Routing infrastructure and protocol design. The routing infrastructure of the network also needs to be considered when developing the network design. For example, will the network use traditional routers only, or some combination that includes Layer 3 switching? Outside of the physical equipment to be implemented, the routing protocols to be used needs to be specified and documented. In a network design using OSPF as its only routing protocol, information about elements such as areas and autonomous systems need to be included. Similarly, the proposed hardware that will be used to implement the design, such as specific switch and router models and specifications, must be fully documented

Security design. The security design of the network can encompass a variety of hardware and software features. The need for and placement of elements like firewalls and intrusion detection systems much be documented, as should the particular hardware and software that will carry out these functions.

Advanced features design. Any advanced features to be implemented on the new or upgraded network need to be carefully documented, ensuring that any proposed equipment provides the capabilities that these features require. For example, a new network might require QoS features for a VoIP implementation, or need to include IP Multicasting capabilities in order to support a streaming media application.

Although most of the network applications, features, and equipment required in a network design fall into one of the categories listed above, these are not the only possibilities that exist. The general rule of thumb to be followed is to be sure that any new or upgraded network element that falls into the scope of the project should be fully documented and diagrammed as appropriate. This information will ultimately be provided to the customer as part of the network design document that will be developed as part of step 6 in the structured network design process.

Author: Dan DiNicolo

Dan DiNicolo is a freelance author, consultant, trainer, and the managing editor of He is the author of the CCNA Study Guide found on this site, as well as many books including the PC Magazine titles Windows XP Security Solutions and Windows Vista Security Solutions. Click here to contact Dan.