Build the Network, Watch It Run
A NetSim Field Guide
Build the Network, Watch It Run
A network is easier to understand when you can watch a packet move, stop, and explain why.
Who this is for
This book is for people learning networking who want to build a topology, configure it with real Linux commands and see what happens. It is also for educators who want to author labs that grade themselves.
It assumes you can use a browser and a terminal. It does not assume you have configured a firewall before.
What NetSim is
NetSim is a network simulator that runs entirely in the browser. You drag hosts, switches, routers, firewalls and access points onto a canvas, cable them, open a terminal on any device, and use a curated subset of real iproute2, nft, iptables, DHCP, DNS and HTTP commands.
There is no backend. Topologies autosave in the browser, export as JSON, and share as compressed URLs.
What it simulates
L2 switching and VLANs, ARP, IPv4 routing with static and RIP routes, ICMP, a TCP handshake, UDP, DHCP, DNS A records, HTTP, stateful firewalling with NAT, and connection tracking. Every packet is an event you can watch.
It does not simulate IPv6, OSPF/BGP, 802.1Q trunking, TLS, real application payloads or performance. It is a teaching and design tool, not a packet-accurate emulator.
How to read it
Part 1 teaches the interface, the topology and the commands. Part 2 tests firewalls and services, and explains the bundled labs. Part 3 shows how to author, distribute and export a lab.
Each chapter ends with an AI exploration, a build or lab task, a summary and a reflection.