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

There are a few ways to run an ansible ad hoc command or the ansible-playbook command against certain target servers. 

• Using the hosts parameter in your playbook (ansible-playbook command only) (this article)
• Using the when parameter in your playbook (ansible-playbook command only)
• Using the -i or --inventory command line option
• Using the --limit command line option

After a clean install of Ansible, the "inventory" directive in ansible.cfg is commented out, like this.

#inventory = /path/to/hosts

In this scenario, the default hosts file is /etc/ansible/hosts and the default hosts file is completely commented out. If you were to issue command ansible all -m ping, the following would be displayed. Likewise, if you were to uncomment the "inventory" directive in ansible.cfg without defining your inventory, the following would be displayed.

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

Typically, target servers are defined in the default hosts file or your own inventory file. Sometimes, the "inventory" directive in ansible.cfg is uncommented and updated to point to the directory where the default hosts file or your own inventory file will be located.

Additionally, 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 a default hosts file or your own inventory file named inventory.yml that contains target systems, perhaps something like this.

windows:
  hosts:
    server1.example.com:
    server2.example.com:
linux:
  hosts:
    server3.example.com:
    server4.example.com:

In this example, the playbook has hosts: all, meaning the playbook would be run against "all" hosts in your default hosts file or your own inventory file.

---
- hosts: all
  tasks:
    - name: create /tmp/foo.txt
      file:
        path: /tmp/foo.txt
        state: touch
...

If you were to instead use hosts: windows, the playbook would only be run against server1 and server2.

Or, if hosts: linux were used, the playbook would only be run against server3 and server4.

---
- hosts: linux
  tasks:
    - name: create /tmp/foo.txt
      file:
        path: /tmp/foo.txt
        state: touch
...

Here is an example of how you could run the playbook against the target servers in two or more groups.

---
- hosts:
  - linux
  - windows
  tasks:
    - name: create /tmp/foo.txt
      file:
        path: /tmp/foo.txt
        state: touch
...

Let's say you want to run the playbook against only the first host. In this scenario you can use - hosts: all[0] to target only the first host.

---
- hosts: all[0]
  tasks:
    - name: create /tmp/foo.txt
      file:
        path: /tmp/foo.txt
        state: touch
...

The ~ character can be used to use a regular expression to match one or more target servers. In this example, the regular expression should match server2.example.com.

---
- hosts: ~.*2.*
  tasks:
    - name: create /tmp/foo.txt
      file:
        path: /tmp/foo.txt
        state: touch
...

In this example, the regular expression should match each host that does not contain "2".

---
- hosts: !~.*2.*
  tasks:
    - name: create /tmp/foo.txt
      file:
        path: /tmp/foo.txt
        state: touch
...