There is a possibility to make your Virtual Machine searchable and accessible only for particular group of IP adresses. This action is available in Horizon dashboard, in "Security Groups" window.

To restrict access:

Enroll Network and choose Security Groups menu at the panel on the left side on the Horizon dashboard.

Click "Create Security Group" icon, and after entering a name (and optionally description), click "Create Security Group" blue button.

Enter in "Manage Rules" menu, and click "Add Rule" icon on the upper right.

Choose protocol and ports you want to secure, set "Direction" as "Ingress" and "Remote" as CIDR. Enter desired IP range or single number allowed to access your VM. Confirm your choice by clicking "Add" button.

After that, choose desired instance and click "Edit Security Group".

Add newly created rule by clicking "+" icon and "Save".