To step through this recipe, you will need the following:

Here's how we would create one CentOS 7.2 machine with 512 MB of RAM and one CPU, in a private network with a fixed IP 192.168.50.11, and a simple shell output:

vm_memory = 512
vm_cpus = 1

Vagrant.configure("2") do |config|

  config.vm.box = "bento/centos-7.2"

  config.vm.provider :virtualbox do |vb|
    vb.memory = vm_memory
    vb.cpus = vm_cpus
  end

   config.vm.define "srv-1" do |config|
     config.vm.provision :shell, :inline => "ip addr | grep \"inet\" | awk '{print $2}'"
     config.vm.network "private_network", ip: "192.168.50.11", virtualbox__intnet: "true"
   end
end

To add a new machine to this network, we could simply duplicate the srv-1 machine definition, as in the following code:

config.vm.define "srv-2" do |config|
     config.vm.provision :shell, :inline => "ip addr | grep \"inet\" | awk '{print $2}'"
     config.vm.network "private_network", ip: "192.168.50.12", virtualbox__intnet: "true"
end

That's not very DRY, so let's take advantage of the Ruby nature of the Vagrantfile to create a loop that will dynamically and simply create as many virtual machines as we want.

First, declare a variable with the amount of virtual machines we want (2):

vm_num = 2

Then iterate through that value, so it can generate values for an IP and for a hostname:

(1..vm_num).each do |n|
    # a lan lab in the 192.168.50.0/24 range
    lan_ip = "192.168.50.#{n+10}"
    config.vm.define "srv-#{n}" do |config|
      config.vm.provision :shell, :inline => "ip addr | grep \"inet\" | awk '{print $2}'"
      config.vm.network "private_network", ip: lan_ip, virtualbox__intnet: "true"
    end
  end

This will create two virtual machines (srv-1 at 192.168.50.11 and srv-2 at 192.168.50.12) on the same internal network, so they can talk to each other.

Now you can simply change the value of vm_num and you'll easily spawn new virtual machines in seconds.

We can optionally go even further, using the following cloning and networking features.

config.vm.provider :virtualbox do |vb|
    vb.memory = vm_memory
    vb.cpus = vm_cpus
    vb.linked_clone = true
end

config.vm.network "private_network", ip: lan_ip, virtualbox__intnet: "iac-lab"