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There
are
two
reasons
that
attract
us
to
do
subnetting.
The
first
is
to
allocate
the
limited
IP
address
space
more
efficiently.
Based
on
table
2.2,
then
by
default
the
Internet
was
limited to Class A to have 16 million hosts, Class B to
have 65,000 hosts, and Class C to
have 254 hosts per network. While there are many networks with more than 254 devices
exist
such
as
big
company,
there
are
very
few
exist
with
65,000
hosts
per
network,
let
alone
16
million. By
this
specification,
any
network
with
more
than
254
hosts,
let
say
500 hosts in a network, would need a Class B allocation and probably waste tens of
thousands of IP addresses.
The second reason for subnetting is that even if a single organization has thousands of
network devices,
operating
all
those
devices
with
the
same
network
ID
would
be
very
inefficient
because
every
department
might
need
different
needs.
Also,
a
single
network
only has a single broadcast domain, thus it will affect the network performance.
For
performance reasons, networks are usually segmented into broadcast domains that are
smaller than even Class C addresses provide.
2.4.2.3 Subnet Masks
In order for subnetting to work, the router must be told which portion of a particular IP
address
belongs
to
which
network.
This can be accomplished by using another 32-bit
number, called subnet mask. Here is the list of default values for subnet masks from each
class.
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