How do you figure out network address and host address from Subnet Mask?

I am having a major problem understanding network subnets and subnets masks.





Mainly how does subnetting actually allow you to create more IPs?





For subnet masks how do you know what you need and how to tell how many bits are used for network and host ?How do you figure out network address and host address from Subnet Mask?
Here are some sites to learn and practice subnetting:





http://easysubnet.com/


http://subnettingquestions.com