Comments
Below are some best website
http://www.joshgentry.com/networking/subnet.htm
http://www.networkclue.com/routing/tcpip/calculating-masks.aspx
one simple answer i could find
Given a Class B Network with subnet mask of 255.255.248.0 and a packet addressed to 130.40.32.16, Take the 2 addresses, write them in binary form, then AND them. The answer is 130.40.32.0
Posted by suresh123