Section 3.2: Subneting
The process of extending the default subnet mask creates a counting range in the octet that the subnet was extended into, which can be used to represent subnetworks. This allows a single Class A, B, or C network to be subdivided into many smaller groups with each group, or subdivision treated as if it were a network itself.
Thus, when we extend the default Class B subnet mask to 255.255.240.0, we do so by extending the subnet mask by 4 bits into the third octet. The number of bits that the subnet mask is extended by represents a counting range for counting the number of subnetworks that new subnet mask can support, using the 2n-2 formula. Thus, the subnet mask 255.255.240.0 subnet mask can support 14 subnets (24-2). In other words, the 65,534 hosts supported by the default subnet mask can now be divided among 14 subnetworks. The number of IP addresses supported by each subnet is called an address range. To calculate the range of addresses for each subnet, we would take the decimal value for the last bit used for the subnet mask as the starting point for the first address in our subnetwork, and then increment that number for each subsequent subnet. In this octet the bit range would be 1111 0000. The last bit in the subnet mask would thus have a decimal value of 16 (0001 0000). Therefore the first IP address in the first subnet address range would be 140.12.16.1. The address ranges for the 14 subnets would be:
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Note: The IP address range for each subnet begins with a 1, as in 140.12.16.1 or 140.12.32.1 and not 140.12.16.0 or 140.12.32.0 as this would be the first address in the subnetwork, and would therefore be the network address. Similarly, the last address in the range ends in 254 and not 255 as the last address would be the broadcast address.