Whatmask is a small C program that will help you with network settings.

Whatmask can work in two modes. The first mode (which is how prior
versions worked) is to invoke Whatmask with only a subnet mask as the
argument. In this mode Whatmask will echo back the subnet mask in four
formats, plus the number of useable addresses in the range.

Netmask Notations supported:

 Name                  Example
---------------------------------
 CIDR                         /24
 Netmask            255.255.255.0
 Hex Netmask           0xffffff00
 Wilcard Bits           0.0.0.255