How to calculate subnet mask?

whatmask - This is a subnet mask notation conversion tool. Or a
Tool for calculating available host address ranges with CIDR
notation input.

[root@vikas Desktop]# whatmask 192.168.4.0/24

------------------------------------------------
TCP/IP NETWORK INFORMATION
------------------------------------------------
IP Entered = ..................: 192.168.4.0
CIDR = ........................: /24
Netmask = .....................: 255.255.255.0
Netmask (hex) = ...............: 0xffffff00
Wildcard Bits = ...............: 0.0.0.255
------------------------------------------------
Network Address = .............: 192.168.4.0
Broadcast Address = ...........: 192.168.4.255
Usable IP Addresses = .........: 254
First Usable IP Address = .....: 192.168.4.1
Last Usable IP Address = ......: 192.168.4.254

[root@vikas Desktop]#

if whatmask command is not found,
just do
yum install whatmask