Security sees benefit since the IP addresses of the host computers on
each subnet are masked by the network address- which means they are invisible
to the outside world. We call this network address translation, or NAT. This
same technique helps conserve IP addresses, since all hosts on the subnet
essentially just use the network IP address during communication.
A Basic Review on Subnetting
If you’re a little rusty on the basics of
subnetting, don’t worry. We’ll cover everything you need to know from beginning
to end. First thing first: we need to learn about the IP basics.
The current version of internet protocol, or IP, is IP version 4. This
IP version allows four octets of data to represent an IP address. Each octet is
considered to be a byte, so there are 8 bits in every octet. Note that in
binary form you can see that there are 8 numbers, each one consisting of a bit.
Finally, each octet is separated by a period, as shown below.
Each IP address is usually represented in decimal form, as seen above
as “192.168.2.1”. However, each IP address is actually used by computers in
binary form. You may have noticed a huge flaw in IP version 4: the amount of
unique IP addresses is limited! To be exact, only 4,294,967,296 unique IP
addresses can be created. This may seem like a large number but keep in mind
every single device in the world needs a unique IP address to communicate with
one another while online.
The IP version 6 protocol was created for when the transition is
needed. Most computers don’t use IPv6 just yet, but in the future it will
inevitably be used. (In case you were wondering, IPv6 will support 2^128 unique
IP addresses… egad!)
Classes
There are three main types of classifications of IP addresses in IPv4:
class A, class B, and class C. There are class D and class E types, but those
are for multicasting and private uses, respectively. Each class differs by the
number of network and host octets it has. Each network octet is for classifying
which network a host is on. More network octets will mean more networks!
Likewise, each host octet specifies a host that can be assigned to the network.
More host octets means more computers per network.
Each class has a certain range that the first octet can be assigned to.
This lets us know to what network class any IP belongs to with ease. Review the
diagram below to see a visual diagram.
Now the bad news: you must memorize each range for each class if you hope
to pass most network exams. Don’t worry! It’s actually easy- just memorize how
we get the numbers, not the numbers themselves! All you have to do is remember n^8
(read as “n” to the eighth power), and how many network portions each class
has. Review the diagram below.
One final note before we move on. Notice how we didn’t use the IP
address 127 for the class A network- that’s because this is loopback address.
We use this for testing configurations on the IP network. Also take note that
there are reserved IP addresses, such as those for private networks. These IP
addresses will not connect to the internet, which is handy if you just want
computers to be connected with each other- and not the whole world.
Subnet Masks
A subnet mask is what we use to assign different amounts of networks
and hosts to an IP network. It essentially divides the IP address into two
parts: the network and host. This is a trading game- the more hosts you have,
the fewer networks you will have. And yet the more networks you have, the fewer
hosts you will have. Review the class A default subnet masks as shown below.
The class B network will need to recognize two network portions, and
two host portions. This can be seen in the below diagram.
Lastly, we have the class C subnet. It is the most commonly used
subnet, so pay special attention to it in the upcoming lecture. You can see a
diagram of the default subnet below.
We’ve laid Down the Foundation, Let’s Build the House!
We just reviewed the basics of IP addresses and
subnet masks. The bad news is, we usually don’t use the default subnet mask at all
when we subnet a network! The good news is, learning how to subnet isn’t too
hard. It will, however, take some practice.
An Example of Subnetting a Class C Network
Subnetting varies in difficulty, depending on which class you are
subnetting. More often than not, you will need to create a subnet for a class C
network. A class C network allows for many networks to be created, but not as
many hosts. In fact, you can create a hefty 2,097,152 networks- but only 254
hosts per network are allowed.
Let’s say we are subnetting a school’s network. We need 5 separate
networks that have 30 computers on each subnet.
First we calculate how many usable subnets we need. Next we will need
to determine how many hosts are required. Both of these values can be calculated
with the following equations:
Two Important Equations to Remember:
- 1. Usable Subnets = (2^n) – 2 , where N = power of bits assigned
- 2. Usable Hosts = (2^n) – 2 , where N = power of bits remaining
Let’s start with usable subnets. Review the diagram below for a visual
example.
If you are wondering what the “-2” part of the equation is for, this is
how we account for the two addresses in each subnet we can’t use. We will
review what these addresses are for more specifically later on. For now, let’s
find out what our usable hosts are!
Alright Already! What’s My Subnet Mask?!
Getting your subnet mask at this point is incredibly
easy. Simply take all of your network bits, and add them up. Look at the
diagram below for a visual guide.
That’s it! You’re done. You have successfully created a subnet mask
that can be used on the school’s network. Keep in mind that every computer must
have the subnet mask set in order for them to be on the same network. Also note
that since we are using a class C network, the first three octets will always
be 255. When subnetting other classes, be sure to keep the network portions in
mind.
Closing Comments
Subnetting may not be fun, but it is required for students to know on
networking exams. That means you should stay away from subnet calculators until
you have grasped the full concept of how to do it yourself. These calculators
should only be used as a time saving tool, not a learning tool or way to cheat
on homework.
With enough practice, subnetting will become much easier. Class A and
Class B networks are a bit trickier, but follow the same example as shown above
and it should be quite easy to accomplish.
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