It uses a proxy. JSLinux emulates a network device and sends ethernet frames to a proxy via websockets. The proxy sends these frames to a Linux TAP device. The rest is a decent firewall configuration.
You might even be able to connect to another emulated JSLinux machine if you know that IP address.
> Can I access to the network from the virtual machine ?
Yes it is possible. It uses the websocket VPN offered by Benjamin Burns (see his blog). The bandwidth is capped to 40 kB/s and at most two connections are allowed per public IP address. Please don't abuse the service.
