So far, we’ve been working with only one server (or host, as Ansible calls it). In reality, you’re going to be managing multiple hosts. The collection of hosts that Ansible knows about is called the inventory.
The default way to describe your hosts in Ansible is to list them in text files, called inventory files. A very simple inventory file might just contain a list of hostnames, as shown in Example 3-1.
ontario.example.com newhampshire.example.com maryland.example.com virginia.example.com newyork.example.com quebec.example.com rhodeisland.example.com
Ansible uses your local SSH client by default, which means that it will understand any aliases that you set up in your SSH config file. This does not hold true if you configure Ansible to use the Paramiko connection plug-in instead of the default SSH plug-in.
There is one host that Ansible automatically adds to the inventory by default: localhost. Ansible understands that localhost refers to your local machine, so it will interact with it directly rather than connecting by SSH.
Although Ansible adds the localhost to your inventory automatically, you have to have at least one other host in your inventory file; otherwise, ansible-playbook will terminate with the error:
ERROR: provided hosts list is empty
In the case where you have no other hosts in your inventory file, you can explicitly add an entry for localhost ...