i have found the IP range 120.76.0.0 - 120.79.255.255 that i want to ban in iptables the way i write x.x.x.x/xx format. Please how to know what that x.x.x.x/xx should be? I have found some online tool where i enter the range and it return it, but unable, these tools like https://www.calculator.net/ip-subnet-calculator.html only have field for single IP and subnet mask.
Update: currently this calculator should work: https://community.spiceworks.com/tools/subnet-calc/ and set "Number of subnets" to 1