FQDN in playbook but not in inventory file - skipping: no hosts matched

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Hi all,

When specifying an host in the playbook itself but not in the inventory file, I get the message 'skipping: no hosts matched.'

When I pass the host via:

ansible-playbook playbook.yml -i hostname,

All is just working fine.

But why is Ansible unable to just pick up this host defined in the playbook without needing an inventory file that specifies exactly the same information about this host?