Subnetting headache looking for the miracle tool

I've been tasked to find an available subnet that can be used for a section of our internal network. I'm not familiar with it enough to not mess it up....Read below.

We have several sites located in different building that are all interconnected on the same Active Directory domain, Cisco routers connecting each site with an MPLS and MetroEther. Each building already has several subnets setup and I have a record of them all.

I'm not familiar enough with this to pick a new one or break down one that has an over abundance of IP addresses.

we're using 192's, 10's 172's etc.

Is there a tool I could use to enter all of my subnets and if spit out an available one?

I only need 30 IP's for the new subnet..

Any help or guidance will be appreciated. Of course these are all internal non public IP address.
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JustInCaseConnect With a Mentor Commented:
And of course cheat sheet.
Both answers provide tools for easier subnetting.
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