Multi-VM.rb

# This shows host only networking in a Multi-VM environment.
#
# The key point in this example is that the web and db sub-VMs
# are put on the same network so they can communicate with each
# other.
Vagrant::Config.run do |config|
  # ...

  config.vm.define :web do |web_config|
    # Attach the web VM to the host only interface with a
    # static IP.
    config.vm.network("my_network", "192.168.48.101")

    # Still forward the port for localhost access on the
    # host machine (this isn't necessary since you can also
    # access via the above IP). This will forward the port
    # on adapter 0 (eth0), whereas the above is setup on
    # adapter 1 (eth1)
    config.vm.forward_port("web", 80, 8080)
  end

  config.vm.define :db do |db_config|
    # Attach the DB VM to the same host only network as the 
    # web server so they can talk to each other.
    config.vm.network("my_network", "192.168.48.102")
  end
end

Single VM.rb

# This example represents single VM host only networking.
# This is useful in the case where someone wants to add a
# single VM to a pre-existing host only networking, and also
# provides a solid base to build up for multi-VM.
Vagrant::Config.run do |config|
  # ...

  # Add to a host only interface. This uses DHCP to get the IP,
  # and attaches the host only interface to adapter 1 (eth1), by
  # default.
  config.vm.network("vboxnet0")

  # Another example. This is still DHCP but instead attaches to
  # adapter 2 (eth2).
  config.vm.network("vboxnet0", :adapter => 2)

  # Another example. This assigns a static IP to the VM while still
  # connecting to a host only interface.
  config.vm.network("vboxnet0", "192.168.48.101")
end