Cisco IOS sandbox
An empty bench, like a new Packet Tracer file. Put devices on it, cable them, then click one to configure it. Pick the cable as Packet Tracer would: straight-through between unlike devices, crossover between like ones, serial between routers, console from a PC's RS 232 to a device. Build freely, or pick a problem and build what it asks for. It is kept in this browser.
Routers
Switches
End Devices
Connections
Drag a router, switch or PC onto the bench, or press one above to place it.
Place a device to begin.