The hierarchical internetwork model, created by Cisco, is a three-layer framework that organizes networks into three distinct layers: core, distribution, and access. Each layer has its own responsibilities for providing services to end devices and servers.

A key aspect of network design is to use switching whenever possible and routing only when necessary. This means that switches should be the primary choice for connecting devices. Another important element is dividing network devices into different zones, which helps separate user access networks from data centers. This separation can be done using routers, switches, or firewalls. Access devices typically connect end devices like VoIP phones, printers, and computers. Networks can be organized by floor or by office.

Core Layer
The core layer acts as the backbone of the network and is usually built with high-performance switches and fiber optic cables. This layer does not handle traffic routing to local area networks (LANs) and focuses solely on fast and reliable packet delivery. It is designed with redundancy to ensure continuous operation. The model starts with two core switches that manage switching and routing, allowing for maximum performance through multiple connections to the distribution layer.

Distribution Layer
The distribution layer connects to the access layer and is mainly focused on switching. It can connect redundantly to both the core and user switches. To achieve the best performance, uplinks to this layer should also consist of multiple connections. Firewalls and Network Address Translation (NAT) can be set up at this layer. Routing between different VLANs and workgroups occurs here, and devices in this layer need to handle a large amount of traffic. In larger networks, multilayer switches are recommended. Redundancy is also important at this layer, as a failure could impact many users. Devices should have backup connections to access layer devices and redundant power supplies.

Access Layer
The access layer, also known as the edge layer, includes hubs and switches that connect client devices to the network. This layer is responsible for ensuring that clients can receive data on their computers and phones. Any device that connects users to the network is considered an access layer device. The architecture of the hierarchical internetwork model developed by Cisco is illustrated in the accompanying figure.