Packet Tracer implements the mechanisms behind a small routed network: Ethernet switching, IEEE 802.1D spanning tree, 802.1Q VLANs, ARP, ICMP, LLDP, static and RIP routing, and RFC 2003 IP-in-IP tunneling. The native CLI carries simulated frames over local UDP sockets; the browser version compiles the same Go model to WebAssembly. It is the current evolution of the earlier Go-Netstack project.
The interactive UI makes each forwarding decision inspectable. A user can play or step through ARP, ICMP, and LLDP events, then inspect interface state, STP roles, MAC and ARP tables, routes, and discovered neighbors at that point in the trace. Built-in scenarios cover switching, VLAN trunks, inter-VLAN routing, route selection, STP failover, and campus topologies.
Custom YAML topologies stay entirely in the browser and can model up to one hundred devices. Building the simulator meant treating protocol state as part of the explanation: ARP cache changes, spanning-tree convergence, split-horizon behavior, and encapsulation decisions need to be visible rather than hidden behind a final successful ping.