Motorola MG7540 User Manual page 44

Note that Parental Control is Disabled by default. You should leave it disabled until
you have set up all desired Whitelists and Blacklists. Enable Parental Control once
the Whitelist and Blacklist settings are complete. Select Enable, and then Save.
If you need to modify the Whitelists or Blacklists, first Disable Parental Control and
Save. Then, Enable Parental Control when the changes are complete, and Save
again.
To set up either a whitelist or a blacklist, you will need to first find the MAC
Address of the device you want to control. On some devices, this may be found on
a label. On others, there may be a way to query the device to display this. Check
your user documentation for that device.
You may also find the device's MAC Address by looking at the client list on your
MG7540. Click Basic Router  DHCP and look at the MAC Addresses in the DHCP
Client List: