It is easy to partition a single IP based network address into smaller addresses; but the question is, how do we know that which IP address falls in which subnet.
To do this we have some ranges and with the knowledge of ranges users can easily identify the Class in which a particular address lies. The three high order bits of the IP address field thus partition the full range of 256 possible 8 bits numbers that can appear in the last octet of an IP address into eight group of 32 addresses as shown.

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However, reserved addresses for network and broadcast must be excluded. The address shown for subnet 0 and 7 must be excluded. The range of numbers that have 00000 in the host portion, address with 0,32,64,96,128, 160, 192and 224 in the last field of the IP address. This means that you must exclude the first address in each of the address ranges noted in the table-12 above.