The previous link was dead. The updated code can be found here (if you just want the end product download the Install file):
How to use the IPv4 Subnet Calculator
The first step is to enter a valid IPv4 (IP) address into the IP Address field.
Next, create the mask. This can be done by specifying the size of the mask (CIDR) into the number box, after the "/", or use the track bar. All mask related controls on the form will stay synchronized.
At this point you have defined the base network and will see the other fields. On the right you will see the binary representation of the fields.
In the tree view you will see the base network and the first two possible subnets.
Clicking on a plus sign will expand that network showing its subnets.
Clicking on a network will show the binary representation for that network and will be indicated by a highlight change.
Right clicking a network will make it the base network.
Once you have a base network defined an automatic subnet plan of the base network can be created by clicking the ‘Plan’ button. This will bring up a new form.
The plan is a list of network sizes (number of hosts in a network) entered into the list. The planner will take care of the broadcast address and whether or not to use the network address as a host. You can also use a growth factor if desired.
When the list is completed click ‘CreatePlan’ and the sub-netting of the base network will be done. If ‘Watch’ is checked you will see the process. If ‘Copy Plan on Close’ is checked the plan will be copied to the clipboard when the plan form is closed. If the base network is not large enough for the list a message will be shown.
2.0 released December 12th, 2013