I'm looking for a tool which will calculate subnet for a specified address range, but unless I'm misunderstanding the popular ones out there, I haven't been able to find one.

So, for example, if I have a VPN road warrior set up, and I'm somewhere where the network is 10.0.0.1/24 but the VPN tunnel network happens to be 10.0.0.1/30? Changing the client network address ...