IP Addressing and Subnetting 241
C network as if it were a network itself. By doing so, a single Class A, B, or C network can be
subdivided into many nonoverlapping subnets.
The needs for subnetting are both technical and administrative, as documented in the following
list:
·
All organizations connected to the Internet (and not using IP address translation) are
required to use IP networks registered with the NIC.
·
IP protocols enforce the following grouping concept: All hosts in the same group must not
be separated by an IP router.
·
A corollary to the grouping concept is this: Hosts separated by an IP router must be in
separate groups.
·
Without subnetting, the smallest group is a single, entire Class A, B, or C network number.
·
Without subnetting, the NIC would be woefully short of assignable networks.
·
With subnetting, the NIC can assign one or a few network numbers to an organization, and
then the organization can subdivide those networks into subnets of more usable sizes.
An example drives these points home. Consider all network interfaces in Figure 5-20, and note
which ones are not separated by a router.
In Figure 5-20, six groupings exist, each of which is a Class B network. Four networks are more
obvious, those being the set of all interfaces attached to each of the four LANs. In other words,
the LANs attached to Routers A, B, C, and D are each a separate network. Additionally, the two
serial interfaces composing the point-to-point serial link between Routers C and D are both in
the same network because they are not separated by a router. Finally, the three router interfaces
composing the Frame Relay network with Routers A, B, and C would not be separated by an IP
router and would compose the sixth network.
If building this network today, the NIC would not assign six separate Class B network numbers,
as shown in Figure 5-20. Instead, you might get one or two Class C networks assigned by the
NIC, with the expectation that you would use subnetting.
Figure 5-21 illustrates a basic subnetting example. (Basic is a term used for purposes in this
book to denote subnetting examples for which the math is easy. More advanced subnetting is
covered later in this section.)
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