site.yml

- hosts: all

  tasks:

  - name: Create a group of all hosts by operating system
    group_by: key={{ansible_distribution}}-{{ansible_distribution_version}}

# the following host group does not exist in inventory and was created by the group_by
# module.

- hosts: CentOS-6.2

  tasks:

  - name: ping all CentOS 6.2 hosts
    ping:

- hosts: CentOS-6.3

  tasks:

  - name: ping all CentOS 6.3 hosts
    ping:

- hosts: MacOSX-10.11.3
  tasks:
    - name: ping all MacOSX-10.11.3 hosts
      ping: