If you are not familar with Solarwinds, you had better start taking a look. They have some of the best freeware networking tools available, in addition to some of the best network monitoring tools on the market (such as Orion NPM). Yes, their tools are extremely expensive because they are meant to be used to monitor or manage a corporate network. Nonetheless, they offer some very useful free tools on their site. Here is a small list of some tools that I use from day to day:
Subnet Calculator <—Helped me with my network design on SEVERAL occasions
Hit up http://www.solarwinds.com for more info and free trials of their premier software.