Network design fundamentals: building blocks, network types and the design plan

What a network is for

A network is a set of devices linked so they can exchange data and share resources, over cables or wireless. Networks carry the things people now take for granted: file sharing, video calls, web browsing, instant messages, email and voice over IP. They shape how we learn, communicate, work and play.

To build any network you need four ingredients:

• End devices: where data starts or finishes, such as PCs, phones, servers and printers.
• Intermediary devices: devices that move data along, such as switches, routers, access points and firewalls.
• Protocols: the agreed rules for communicating, such as Ethernet, IP, TCP and HTTP.
• Media: the path itself: copper, fibre or radio.

Three ways to classify a network

Physical versus logical topology

The physical topology is how devices are actually cabled. The logical topology is the path data really takes. They can differ: devices may be cabled in a star to a central device, yet pass data around in a logical ring. Always check both when you document or troubleshoot a network.

Internet, intranet and extranet

• Internet: the worldwide network of networks that links millions of organisations and devices.
• Intranet: a private network for an organisation's own staff to share information and work together.
• Extranet: part of an intranet opened, with controlled access, to trusted outsiders such as customers, suppliers or partners.

Why every network needs a design plan

A design plan is the blueprint for a robust, efficient and secure network. It records the architecture, topology, protocols and technologies the organisation needs. By weighing scalability, performance, reliability, security, manageability and cost at the start, the plan makes sure the network supports today's needs and can grow with tomorrow's.

Rule of thumb: problems fixed on paper cost minutes; problems fixed after installation cost days and money.

Real-world example: planning a new community clinic

A charity opened a two-floor community health clinic with 25 staff, six consulting rooms and a public waiting area. Before buying anything, the network designer wrote a short design plan. It listed the end devices (PCs, label printers, IP phones, a few medical devices and guest Wi-Fi), the intermediary devices (one router, two access switches and four wireless access points) and the media (Cat 6 to every room and a fibre link between floors).

The plan described a physical star topology on each floor and a logical design with separate networks for staff, medical devices and guests. Patient records lived in a cloud system, so the clinic needed a reliable internet link with a 4G backup rather than local servers. Writing it down first helped the charity compare quotes fairly and avoid buying switches without enough Power over Ethernet ports for the phones and access points.