is 255.255.0.0 or /16. The purpose of the / notation is to
specify how many bits in the subnet mask are dedicated
to the network ID. Network addresses can be subdivided
further to create subnets. This is accomplished by using
variable-length subnet masking (VLSM) to indicate how
many bits in an IPv4 address are dedicated to the
network ID and how many bits are dedicated to the host
ID. As an example, the 10.0.0.0 private network can be
further subdivided, and new subnets can be created out
of it. Instead of having one network with more than 16
million hosts, which is not helpful in real life, 65,536
subnets with 254 hosts—each similar to 10.0.0.0/24,
10.0.1.0/24, 10.0.2.0/24, and so on—can be created out
of the 10.0.0.0/8 network. This is done by borrowing 16
bits from the 24 bits that are dedicated to host IDs and
transforming them into network ID bits. By applying a
different subnet mask—using /24 instead of /8—we can
use VLSM and CIDR notation to adjust the host portion
of the network address.
Let’s take a practical example and create new subnets for
a fictitious enterprise called Acme, Inc. The network
administrator is planning on using the 192.168.0.0/24
network to address all the devices that are connected to
the Acme, Inc. enterprise network. As you would expect,
there are several departments in the Acme, Inc. world,
and there are different numbers of employees in each
department. Let’s assume that the total number of
departments is 8 and include the typical corporate
organization: engineering, support, human resources,
finance and accounting, and so on. Acme, Inc. is still a
small company, and there are not more than 20 devices
connected to the network in each of the 8 departments.
The network administrator would like to segment each
department into its own network so that security can be
more easily enforced by using Layer 3 access control lists
(ACLs) and also to limit the broadcast domain for each
department. The 192.168.0.0/24 CIDR notation
indicates that for this subnet, there are 24 bits reserved