How it works
A network is designed backward from what it has to support: how many devices, what they need to reach, which groups should stay apart, and what has to keep working when something fails.
Turn a description into requirements before choosing equipment. Counting devices and grouping them by who needs to reach what comes before any product decision.
Common error
Designing for today exactly is a design that fails next year. Leaving room is part of the requirement, not an extravagance.
Worked example
Plan an installation before choosing equipment.
Evidence
Staff today: 18
Staff growth: 8
Printers: 2
Gateway interface: 1
Guests: separate wireless networkThe staff subnet must accommodate at least 18 + 8 + 2 + 1 = 29 addresses under these requirements. Plan separate guest access, suitable uplink capacity, power, and policy boundaries. Equipment selection follows the address and service requirements rather than preceding them.
Exercises
3.1.1
A group has 40 endpoints and expects 25% growth. How many endpoint addresses should the plan reserve before adding infrastructure addresses?
Show solution
Reserve 40 + 0.25 * 40 = 50 endpoint addresses. Gateway and other infrastructure addresses must be added separately.
3.1.2
List three non-device-count requirements for a classroom network.
Show solution
Examples include required application access, guest isolation, coverage in occupied areas, expected traffic capacity, recovery expectations, power availability, and maintainable documentation. State measurable requirements where possible.