Tasks are run against target servers. Some Ansible documentation refers to the target servers as "hosts".

After a clean install of Ansible, the default hosts file is /etc/ansible/hosts. The default hosts file is completely commented out. If you were to issue command ansible all -m ping, the following would be displayed.

[WARNING]: provided hosts list is empty, only localhost is available. Note that the implicit localhost does not match 'all'

Ansible uses inventory plugins to parse inventory. The ansible-doc command can be used to list the inventory plugins that can be used with the version of Ansible you are using.

~]$ ansible-doc --type inventory --list
ansible.builtin.advanced_host_list Parses a 'host list' with ranges                                                                                                                                                                 
ansible.builtin.auto               Loads and executes an inventory plugin specified in a YAML config                                                                                                                                
ansible.builtin.constructed        Uses Jinja2 to construct vars and groups based on existing inventory                                                                                                                             
ansible.builtin.generator          Uses Jinja2 to construct hosts and groups from patterns                                                                                                                                          
ansible.builtin.host_list          Parses a 'host list' string                                                                                                                                                                      
ansible.builtin.ini                Uses an Ansible INI file as inventory source                                                                                                                                                     
ansible.builtin.script             Executes an inventory script that returns JSON                                                                                                                                                   
ansible.builtin.toml               Uses a specific TOML file as an inventory source                                                                                                                                                 
ansible.builtin.yaml               Uses a specific YAML file as an inventory source

For example, ansible.cfg may have the following.

[inventory]
enable_plugins = ansible.builtin.host_list, ansible.builtin.yaml, ansible.builtin.ini

For example, the yaml inventory_plugin allows you to define target servers in a YAML default hosts file or your own inventory file. For example, let's say you have an default hosts file or your own inventory file named inventory.yml that contains target systems, perhaps something like this.

all:
  hosts:
    server1.example.com:
    server2.example.com:
    server3.example.com:
    server4.example.com:

Here is how you can run the example.yml playbook using the target systems specified in inventory.yml.

ansible-playbook example.yml --inventory /path/to/inventory.yml

There are multiple ways to define your inventory:

• Append managed nodes to the default hosts file (/etc/ansible/hosts) on your Ansible control node
• Create an alternative hosts file, also known as a static inventory file on your Ansible control node
• Create a dynamic inventory file on your Ansible control node
• Update the "inventory" directive in ansible.cfg to point to the location of the default hosts file or your own inventory file
• Create a static inventory in Ansible Tower
• Create a smart inventory in Ansible Tower