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

• 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

Let's say your default hosts file or your own inventory file contains the following. Refer to Ansible - Inventory groups (all | ungrouped | other).

all:
  children:
    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 every managed host (e.g. target systems) in your default hosts file or your own inventory file.

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: all
  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 hosts. 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
...