I have some questions about subnets, I don't understand what they do or how they work. My home network uses 255.255.255.0 as the subnet mask, and the network at my school uses 255.255.0.0. Why are ...

For example, if there is a class C IP, its Subnet Mask would be 255.255.255.0 and the Subnet Prefix Lenght would be 24 as there are 24 network bits.

I set up a l2tp over ipsec vpn for a mac os x client. I can connect fine and I get an address, but the subnet mask is wrong (and corresponds to the class..).