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IP Subnet Addressing ("Subnetting") Concepts
The
original “classful” IP addressing scheme
conceptually divides a large internetwork into a simple two-level hierarchy:
many networks of different sizes, each of which contains a number
of hosts. The system works well for smaller organizations that
may connect all their machines in a single network. However, it lacks
flexibility for large organizations that often have many subnetworks,
or subnets. To better meet the administrative and technical requirements
of larger organizations, the “classful” IP addressing system
was enhanced through a technique known as subnet addressing,
or more simply, subnetting.
In this section I describe the concepts
and general techniques associated with IP subnet addressing. I begin
with an overview of subnetting, including a discussion of the motivation
for the system and its advantages. I discuss how the traditional “two-level”
method for dividing IP addresses becomes “three-level” for
subnetting. I talk about subnet masks and how they are used in calculations
for addressing and routing. I discuss the default subnet masks used
to represent the “classful” Class A, B and C networks in a
subnetting environment, and then how custom subnet masks are used for
classes A, B and C. I then discuss subnet identifiers and general concepts
behind determining subnet and host addresses in a subnet environment.
I provide summary tables for subnetting Class A, B and C networks. I
conclude with a brief discussion of Variable Length Subnet Masking (VLSM),
an enhancement of “conventional” subnetting that improves
its flexibility further.
 Note: I provide a great deal of coverage of subnetting, because understanding it is an important part of learning about how IP addresses work, and hence, how TCP/IP functions. However, I need to emphasize something that far too few references mention: the technique is today considered mostly historical. The reason is that subnetting is still based on “classful” addressing, which has been replaced by classless addressing on the Internet. The concept of a subnet and subnet mask has certainly not disappeared altogether, but the idea of being assigned a Class A, B or C Internet address block and then explicitly subnetting it is no longer relevant.
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 Related Information: This is the first of two sections dedicated to IP address subnetting. The second section follows this one and describes the step-by-step process for subnetting using examples. If you find that after reading this concepts section that you don't quite understand subnetting, try reading the example-based section and you may find that it helps make it all “click”. On the other hand, if you are already somewhat familiar with subnetting, you may find that you can skip this concepts section and just go through the step-by-step examples. You will find much more in that section in the way of “gory details” of subnet mask, subnet address and host address calculations. Putting the practical details there allows this section to concentrate on concepts without getting too bogged down in numbers.
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 Background Information: Understanding subnetting requires a certain familiarity with binary numbers and how they are manipulated. This includes the concept of using boolean operators such as AND to “mask” binary digits. If reading that last sentence made you go “huh?” I strongly recommend reviewing the background section on computing mathematics before you proceed.
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