By default, the firewall for each service allows access to all IP addresses. To restrict traffic, change each service to allow traffic only from your management subnet. You might also deselect some services if your environment does not use them.
You can use the vSphere Web Client, vCLI, or PowerCLI to update the Allowed IP list for a service. By default, all IP addresses are allowed for a service. This task describes how to use the vSphere Web Client. See the topic on managing the firewall in vSphere Command-Line Interface Concepts and Examples at https://code.vmware.com/ for instructions on using the vCLI.