2  Build a Topology

A topology is a set of devices, links and addresses that can be reasoned about.

2.1 The Concept First

NetSim’s device palette has six types:

  • Host: one interface.
  • Server: one interface, with service editors.
  • Switch: eight ports, L2 forwarding and VLANs.
  • Router: four interfaces, IPv4 forwarding.
  • Firewall: four interfaces and nftables base chains.
  • Wi-Fi AP: a bridge with wireless client links.

Drag a device onto the canvas, or click to drop one. Cable two devices by dragging from a dot on one to a dot on the other.

Addressing can be done in two places: the Inspector fields or a terminal. Both use the same engine, so the result is identical.

The starting topology is a claim about the network. Leave unconfigured what the student must configure. Configure what the scenario provides.

2.2 Understanding Through a Model Railway

A model railway is not a real railway. It is a track plan, a set of trains and a power supply that lets you test a timetable.

The topology is the track plan. The addresses and routes are the timetable.

2.3 Discovering Topology with Your AI Partner

Exploration 1: Choose the Devices

Ask the AI to translate a scenario into devices.

Scenario: two subnets, each with two hosts, joined by a router, with a
firewall between the subnets and an internet host.

List the devices, interfaces and links. Mark which device needs a route
and which needs a firewall rule.

A good answer separates the topology from the configuration.

Exploration 2: Plan the Addressing

Ask for an address plan.

Plan IPv4 addressing for two LANs, a transit link and an internet host.
Use private ranges. For each device and interface, give the address,
prefix and default gateway.

The plan is the contract before the commands.

Exploration 3: Find the Minimum Topology

Ask what can be removed.

Here is a topology: [paste it]
Remove devices and links that do not change the question being tested.
Explain what each removal changes.

A smaller topology is easier to debug and grade.

2.4 From Concept to Practice

A topology record:

Device Type Interfaces Addresses Role
pc1 Host eth0 192.168.1.10/24 LAN client
sw1 Switch 8 ports none L2 segment
r1 Router eth0, eth1 .1 on each LAN Gateway
fw1 Firewall 4 ports outside/inside Policy
web Server eth0 203.0.113.10/24 Public service

2.5 Build Task

Build a two-host, one-switch network. Address both hosts, cable them and ping. Then add a router and a second subnet.

2.6 Classroom Connection

A topology that fits on one screen is easier to discuss. Start small; add one device only when the question needs it.

2.7 Practice Exercises

Exercise 2.1 (Level 1). Name the six device types. Exercise 2.2 (Level 2). Build a two-host network. Exercise 2.3 (Level 3). Plan addressing for two LANs. Exercise 2.4 (Level 4). Add a router and default routes. Exercise 2.5 (Level 5). Reduce a topology to the minimum that tests the question.

2.8 Chapter Summary

  • A topology is devices, links and addresses.
  • The palette covers hosts, servers, switches, routers, firewalls and APs.
  • Address in the Inspector or the terminal; the engine is the same.
  • Leave the learning to the student.
  • Start small.

2.9 Reflection

Before moving on, ensure you:

2.10 Your Learning Journal

  1. Which device was easiest to miswire?
  2. Where did the addressing plan help?
  3. What would you remove next time?

2.11 Next Steps

The topology is built. The next chapter configures it with real syntax.