CompTIA A+ Certification All-in-One Exam Guide

“255,” the class system is long gone. Because the subnet mask numbers are
binary, you can make a subnet with any number of ones in the subnet mask.
The current system is called Classless Inter-Domain Routing (CIDR) and
it works easily in binary, but a little less prettily when you show the numbers
in the octets. A quick example should suffice to illustrate this point.
A subnet mask of 255.255.255.0 translates into binary as such:
11111111.11111111.11111111.00000000
With CIDR, network techs refer to the subnet mask by the number of ones
it contains. The preceding subnet mask, for example, has 24 ones. Jill the tech
would call this subnet a /24 (whack twenty-four). As you’ve seen already, a
/24 network ID offers up to 254 host IDs.
If you want a network ID that enables more host IDs, buy one that has a
subnet mask with fewer ones, like this one:
11111111.11111111.11110000.00000000
Count the ones. (There are 20.) The ones mask the network ID. That
leaves 12 digits for the host IDs. Do the binary math: 2 12 – 2 = 4094 unique
addresses in a single /20 network ID.
When you change the binary number—11110000—to an octet, you get the
following:
255.255.240.0
It might look a little odd to a new tech, but that’s a perfectly acceptable
subnet mask. The binary behind the octets works.
From a practical standpoint, all you must know as a tech is how to set up a
computer to accept an IP address and subnet mask combination that your
network administrator tells you to use.
Interconnecting Networks
Figure 19-1 shows a typical LAN you might find in a home or a small office.
Every device on this network has an IP address of, in this example,
192.168.4.x, where x must be a unique value between 1 and 254 (0 and 255
are reserved). Given that every IP address in this group has the same three