There are two versions avaiable. The links here go to the version that runs in a standalone window and works on Mac OSX and OS9. There is a command-line version for OSX on the website.
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 three formats, plus the number of useable addresses in the range.
Netmask notation is pretty much the standard old-school way of doing it. It is supported by most systems (Un*x, Win, Mac, etc.).
Wilcard bits are similar to the netmask, but they are the logical not of the netmask. This notation is used by a number of popular routers (and nobody knows why...)
To use Whatmask in the original mode simply type "whatmask " The notation can be in any of the three formats and Whatmask will automagically figure out what it is and display all three notations.
To find out more about subnets and netmasks see the References section below.
To use Whatmask in its second mode execute Whatmask with any ip address within the subnet, followed by a slash ('/'), followed by the subnet mask in any format. (e.g. 192.168.0.23/255.255.255.224, or 192.168.0.23/27)
Whatmask will echo back the following: The netmask in the following formats: CIDR, Netmask, Wildcard Bits The Network Address The Broadcast Address The number of Usable IP Addresses The First Usable IP Address The Last Usable IP Address (Whatnet assumes that the Broadcast address is the highest address in the subnet. This is the most common configuration.)
What's new in this version:
Added support for hex netmasks Minor bugfixes / cleanup