As mentioned in the previous recipe, Routers along with an external network can be used to provide VMs with access to the Internet. With the External Network set as a gateway on the Router, the VMs can access the Internet from their private IP address. However, the VM cannot be reached from the Internet using the private IP address.
What happens if you are running a web server in your VM? It may be important to allow the users to access the web server from the Internet. OpenStack extends the concept of the Router and external Network and supports the floating IP addresses that can be used to connect to a VM from the Internet.