If you are not familiar dynamic inventory, check out Ansible - Getting Started with Dynamic Inventory.

This assumes you have the host_list plugin in ansible.cfg enabled, like this.

[inventory]
# enable inventory plugins, default: 'host_list', 'script', 'yaml', 'ini'
enable_plugins = host_list, yaml, ini

The ansible command with the --list-hosts and -vvv flags can be used to determine if the host_list plugin is being used.

AVOID TROUBLE

When targetting a single managed node, you must include a comma after the managed node hostname

When targetting two (or more) managed nodes, including a comma after the last managed node is optional

ansible all -i "server1.example.com," --list-hosts -vvv

If you get something like this, this suggests you did not define host_list in ansible.cfg properly.

[WARNING]: Unable to parse server1.example.com, server2.example.com as an inventory source

If the following is returned, the host_list plugin is being used.

Parsed server1.example.com, inventory source with host_list plugin

Something like this should be returned.

  hosts (2):
    server1.example.com
    server2.example.com

Or, the ansible-inventory --list command can be used.

ansible-inventory -i "server1.example.com, server2.example.com" --list

Or, the ansible-inventory --graph command can be used.

ansible-inventory -i "server1.example.com, server2.example.com" --graph