u/tomiczech7

I teach Security+ courses, and students without a networking background kept struggling with the networking part. I tried pointing them to Packet Tracer and GNS3, but those are built for people who want to go deep — CLI configs, endless detail. For someone who just needs to understand how things work, they're more of a barrier than a help.

So I built my own thing: NetworkSim (networksim.app) — a network topology simulator that runs entirely in the browser. No install, no sign-up, no ads, free.

What it does:

drag & drop topology builder (switches, firewalls, IDS/IPS, load balancers, hosts)
firewall zones and ACLs with proper first-match evaluation
traffic path simulation — pick source/destination and see exactly where a packet passes or gets dropped, hop by hop
blast-radius: pick a compromised host and see how far an attacker can actually get with your current segmentation
failure testing: what happens when a key component dies, plus a full reachability matrix

To be clear about what it's not: it won't replace Packet Tracer or GNS3, and it's not trying to. There's no CLI, no real protocol stack — it's a learning tool for building intuition about segmentation, firewall rules, and traffic flow. Students, beginners, homelabbers sketching their network before buying gear — that's the audience.

Full disclosure: this started as an AI-assisted side project that my students stress-tested in class (they found plenty of bugs, most are fixed now). I keep developing it in my free time.

I'd genuinely appreciate feedback — what's confusing, what's missing, what would make it actually useful for you?

NetworkSim v0.2 is out!

I built a free browser-based network simulator for people who find Packet Tracer/GNS3 overkill