How to calculate the NETMASK? (Access Guard)

If you want to allow/block a range of IP address, as like 192.168.1.0~192.168.1.254, we need the following filled:
Choose "NETMASK/PREFIX LENGTH"
IP(IPV4/IPV6):192.168.1.1
NETMASK/LENGTH:255.255.255.0

If you want to allow/block more IP address, as like 192.168.0.1~192.168.3.254 we need the following filled:
Choose "NETMASK/PREFIX LENGTH"
IP(IPV4/IPV6):192.168.1.1
NETMASK/LENGTH:255.255.252.0

or visit following calculator for difference IP range.
http://www.aboutmyip.com/AboutMyXApp/SubnetCalculator.jsp
http://ipcalc.nmonitoring.com/