There are a number of different utilities called ipcalc, almost all of which perform the same basic task. These are handy calculators for converting from CIDR to traditional IP notation and determining network and broadcast addresses.
A short perl script, this prints out alll the information you would want to know about an IP address. It defaults to print colorized output, and comes with its own CGI (shown running here).
For those who perform all operations and research through a web browser, a DHTML calculator should do the trick.
You can run this ipcalc, which features hexadecimal as well as decimal output, on your PDA.
RedHat has created their own ipcalc utility which prints out a shell variable assignment command instead of simply the requested piece of information. In the startup scripts, RedHat evals this variable assignement into existence. Despite this shortcoming, it is a useful tool and is documented in its manpage (part of the initscripts RPM).
Doubtless, there are a large number of other IP calculators available to ease the job of the network administrator. The above tools are meant as a brief summary of some of the offerings.