For Example the IP Address is 192.168.1.125
For human being it is easier to remember the decimal number rather then the binary code.Fot that reson we use the decimal to represent the ip address.but the binary represtation is important to detemine in which class this ip belong with.IP address always assigne according to class.i mean for ip addresses there is a standard that apply according to your requirement.IP classes define how many network ids and how many host this network actually has.
Now we discuse ip classes
Historically, RFC1700 grouped the unicast ranges into specific sizes called class A, class B, and class C addresses. It also defined class D (multicast) and class E (experimental) addresses, as previously presented. The unicast address classes A, B, and C defined specifically-sized networks as well as specific address blocks for these networks, as shown in this figure.
how we get the number of network and number of host per network.see the following table
Note: Class A addresses 127.0.0.0 to 127.255.255.255 cannot be used and is reserved for loopback and diagnostic functions.