Vagrant V2 configuration version, example with new syntax of port forwarding

Configuration versions are the mechanism by which Vagrant 1.1+ ("2") is able to remain backwards compatible with Vagrant 1.0.x Vagrantfiles, while introducing dramatically new features and configuration options.
http://docs.vagrantup.com/v2/vagrantfile/version.html

From my point of view this is the only way to use multiple-machines setup via public/private networks (config.vm.network)