IP address identifies both host and the network where that host resides; the IP address class system gives a clue to distinguish both network and host portion of the address. But the address class system is too inflexible to do the job alone. In the real world, networks come in all sizes, and many networks are divided into smaller units. Furthermore, the world is running out of class-level networks. ISPs and network admins need a flexible means to subdivide a class-level network so that datagrams arrive at routers serving a smaller address space. Subnetting lets you break the network into smaller units called subnets. The concept of a subnet originally evolved around the address class system, and subnetting is best explained in the context of Class A, B and C networks.
Find it difficult to calculate and manage networks and subnets? SubnetCalc is a tool to help you with it! With just the IPv4 address, you can find what you need for a network! It is also intended to be a teaching tool. Not more than 1024 subnets are viewable.