RE : Chapter 1 - SUBNETTING

i) Subnetting is a set of techniques that you can use to efficiently divide the address space of a unicast address prefix for allocation among the subnets of an organization network.
ii)The fixed portion of a unicast address prefix includes the bits up to and including the prefix length that have a defined value. The variable portion of a unicast address prefix includes the bits beyond the prefix length that are set to 0.
iii)Example of subnet : 131.107.192.0/18 (Class B)
The key information in this chapter is the following:

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Subnetting is a set of techniques that you can use to efficiently allocate the address space of one or more unicast address prefixes among the subnets of an organization network.
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To determine the subnet prefix of an IPv4 address configuration in prefix length notation (w.x.y.z/n), retain the n high-order bits, set all the remaining bits to 0, and then convert the result to dotted decimal notation. To determine the subnet prefix of an IPv4 address configuration in subnet mask notation, perform a bit-wise logical AND between the IPv4 address and its subnet mask.
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When determining the number of host ID bits in an IPv4 address prefix to use for subnetting, choose more subnets over more hosts per subnet if you have more possible host IDs than are practical to use on a given subnet.
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To subnet an IPv4 address prefix, use either binary or decimal methods as described in this chapter to enumerate the subnetted address prefixes and the ranges of usable IPv4 addresses for each subnet.
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Variable length subnetting is a technique of creating subnetted IPv4 address prefixes that use prefix lengths of different sizes.
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To subnet an IPv6 global address prefix, use either hexadecimal or decimal methods as described in this chapter to enumerate the subnetted address prefixes.