Webmin allowed IP addresses

As an extra security layer, we recommend that you limit access to Webmin to local IP addresses.

Enter IP addresses as a comma-separated list. IP subnets can be entered by typing address/mask. The mask can be either a network mask or a plain number, specifying the number of 1's at the left side of the network mask. Thus, a mask of 24 is equivalent to 255.255.255.0.
You can use this Wolfram Alpha widget to calculate which net mask corresponds to which address range.

Disable Webmin session authentication

To avoid Webmin session cookies, go to Webmin Configuration > Authentication and disable session authentication.