1  A Network in the Browser

The point of a simulator is not to look like a network. It is to explain what a network did.

1.1 The Concept First

NetSim runs entirely in the browser. It has no backend, no containers and no real kernels. It is a discrete-event simulator: devices, links and packets are objects in TypeScript, and every packet is an event on a clock.

That design gives three things a real network cannot:

  • Determinism: the same topology and the same actions produce the same result.
  • Inspectability: every packet can be paused, stepped and explained.
  • Portability: it runs on a locked-down laptop or a Chromebook with no install.

The trade-off is scope. NetSim simulates a curated subset of IPv4 networking: L2 switching, ARP, static and RIP routing, ICMP, TCP handshakes, UDP, DHCP, DNS, HTTP, stateful firewalls and NAT. It does not simulate IPv6, OSPF/BGP, TLS, real payloads or performance.

It is a teaching and design tool. It is not a packet-accurate emulator.

1.2 Understanding Through a Flight Simulator

A flight simulator does not reproduce every bolt in the aircraft. It reproduces the instruments and the decisions that matter for a flight lesson.

NetSim reproduces the decisions that matter for a networking lesson: addressing, routing, switching, firewalls and services.

1.3 Discovering the Tool with Your AI Partner

Exploration 1: Separate the Model from the World

Ask the AI to list what the simulator can and cannot prove.

NetSim simulates ARP, IPv4 routing, ICMP, TCP, UDP, DHCP, DNS, HTTP,
stateful firewalls and NAT.
It does not simulate IPv6, OSPF/BGP, TLS, real payloads or performance.

For each capability, write one question the simulator can answer and one
it cannot. Do not claim the simulator proves more than its model.

A good answer keeps the model boundary visible.

Exploration 2: Find the Explainability

Ask why a deterministic simulator can explain a drop.

A real kernel drops a packet silently. A deterministic simulator can name
the rule, the interface and the exact event. Explain why that matters for
teaching and for firewall design.

The explanation is the pedagogical advantage.

Exploration 3: Choose the Use

Ask which tasks fit.

List five networking tasks. For each, say whether NetSim is a good fit,
a partial fit or the wrong tool. Include production performance testing,
firewall rule design, protocol learning and incident response.

The answer is a scope decision, not a sales pitch.

1.4 From Concept to Practice

A tool-fit record:

Task Fit Why
Learn ARP Strong Visible packets and tables
Design a firewall Strong Rule model and verdict log
Test throughput Poor No performance modelling
Learn OSPF Poor Not simulated
Teach subnetting Strong Immediate feedback

1.5 Build Task

Open the demo topology. Pause the clock, step through one ping, and name every device the packet touched.

1.6 Classroom Connection

The simulator is a glass box. Students can see the packet, the table and the rule. That visibility is the lesson.

1.7 Practice Exercises

Exercise 1.1 (Level 1). List three things NetSim can explain that a real network cannot. Exercise 1.2 (Level 2). Name three things NetSim does not simulate. Exercise 1.3 (Level 3). Classify five tasks as good, partial or wrong fit. Exercise 1.4 (Level 4). Explain the trade-off between determinism and realism. Exercise 1.5 (Level 5). Write a one-paragraph scope statement for using NetSim in a course.

1.8 Chapter Summary

  • NetSim is a browser-based discrete-event simulator.
  • It is deterministic, inspectable and portable.
  • It simulates a curated IPv4 subset, not everything.
  • The explanation of a drop is the pedagogical advantage.
  • Choose the tool for the question.

1.9 Reflection

Before moving on, ensure you:

1.10 Your Learning Journal

  1. What did the animation make visible?
  2. What is outside the model?
  3. Where would you still use a real lab?

1.11 Next Steps

The tool is scoped. The next chapter builds a topology.