Subnetting a Class C Network
Subnetting a Class C
network address
Why subnetting?
By creating smaller IP networks (instead of having one large network),
we can obtain better security, smaller collision and broadcast domains, and
greater administrative control of each network. Think of a network like
streets in a city. Each house on this network is known by the street and by
the address. Think of the addresses on the houses as the hardware addresses
of a host. For IP to communicate with a host, the IP address must be known,
and the router connected to the network on which this host is located must
also know the hardware address of the house.
What if a city didn't have many blocks but just one long street? The mailman
would go crazy trying to get the mail delivered to each house correctly
because he would have to know the address of every house. It's the same
scenario with IP. By creating smaller networks, we can more effectively get
data to each host.
Subnetting a Class C network
So you understand why you want to subnet, but how do you do it? Your
goal is to look at an IP address and subnet mask of a host and then
determine three things quickly:
 The subnet the host is located in
 The broadcast address of the subnet
 The valid host range of the subnet used to
configure hosts
Once the subnet is determined, the broadcast address must be found. Why?
Because these are not valid host addresses and cannot be assigned to host
configurations. Also, by determining the subnet and broadcast addresses, we
can easily determine the host addresses because the valid host range is
always the numbers between the subnet address and the broadcast address.
If we use the default subnet mask with a Class C network address, then we
already know that three bytes are used to define the network and only one
byte is used to define the hosts on each network.
The default Class C mask is: 255.255.255.0. To make smaller networks, called
subnetworks, we will borrow bits from the host portion of the mask. Since
the Class C mask only uses the last octet for host addressing, we only have
8 bits at our disposal. Therefore, only the following masks can be used with
Class C networks (Table A).
Table A 
Mask 
Binary 
# Subnet bits 
# Host bits 
Subnets 
Hosts 
255.255.255.128 
10000000 
1 
7 
2 
126 
255.255.255.192 
11000000 
2 
6 
2 
62 
255.255.255.224 
11100000 
3 
5 
6 
30 
255.255.255.240 
11110000 
4 
4 
14 
14 
255.255.255.248 
11111000 
5 
3 
30 
6 
255.255.255.252 
11111100 
6 
2 
62 
2 
Class C masks
You can see in Table A that the bits that are turned on (1s) are used for
subnetting, while the bits that are turned off (0s) are used for addressing
of hosts. You can use some easy math to determine the number of subnets and
hosts per subnet for each different mask.
To determine the number of subnets, use the 2^{x}2, where the x
exponent is the number of subnet bits in the mask.
To determine the number of hosts, use the 2^{x}2, where the x
exponent is the number of host bits in the mask.
To determine the mask you need for your network, you must first determine
your business requirements. Count the number of networks and the number of
hosts per network that you need. Then determine the mask by using the
equations shown aboveāand don't forget to factor for growth.
For example, if you have eight networks and each requires 10 hosts, you
would use the Class C mask of 255.255.255.240. Why? Because 240 in binary is
11110000, which means you have four subnet bits and four host bits. Using
our math, we'd get the following:
2^{4}2=14 subnets
2^{4}2=14 hosts
Many people find it easy to memorize the Class C information because Class C
networks have few bits to manipulate. However, there is an easier way to
subnet.
Easy subnetting
Instead of memorizing the entire table (Table A), it's possible to
glance at a host address and quickly determine the necessary information if
you've memorized key parts of the table. First, you need to know your
binarytodecimal conversion. Memorize the number of bits used with each
mask that are shown in Table A. Second, you need to remember the following:
256192=64
256224=32
256240=16
256248=8
256252=4
Once you have the two steps memorized, you can begin subnetting. Our first
example will use the Class C mask of 255.255.255.192. Ask five simple
questions to gather all the facts:
 How many subnet bits are used in this mask?
 How many host bits are available per subnet?
 What are the subnet addresses?
 What is the broadcast address of each subnet?
 What is the valid host range of each subnet?
You already know how to answer questions one and two. To answer question
three, use the formula 256subnetmask to get the first subnet and
your variable. Keep adding this number to itself until you get to the subnet
mask value to determine the valid subnets. Once you verify all of the
subnets, you can determine the broadcast address by looking at the next
subnet's value. The broadcast address is the number just before the next
subnet number. Once you have the subnet number and broadcast address, the
valid hosts are the numbers in between.
Here are the answers using 255.255.255.192:
 How many subnet bits are used in this mask?
Answer: 2
2^{2}2=2 subnets
 How many host bits are available per subnet?
Answer: 6
2^{6}2=62 hosts per subnet
 What are the subnet addresses?
Answer: 256192=64 (the first subnet)
64+64=128 (the second subnet)
64+128=192. However, although 192 is the subnet mask value, it's not a
valid subnet. The valid subnets are 64 and 128.
 What is the broadcast address of each subnet?
Answer: 64 is the first subnet and 128 is the second subnet. The
broadcast address is always the number before the next subnet. The
broadcast address of the 64 subnet is 127. The broadcast address of the
128 subnet is 191.
 What is the valid host range of each subnet?
Answer: The valid hosts are the numbers between the subnet number and
the mask. For the 64 subnet, the valid host range is 64126. For the 128
subnet, the valid host range is 129190.
Let's do a second example using the Class C mask of
255.255.255.224. Here are the answers:
 How many subnet bits are used in this mask?
Answer: 3 bits or 2^{3}2=6 subnets
 How many host bits are available per subnet?
Answer: 5 bits or 2^{5}2=30 hosts per subnet
 What are the subnet addresses?
Answer: 256224 =32, 64, 96, 128, 160 and 192 (Six subnets found by
continuing to add 32 to itself.)
 What is the broadcast address of each subnet?
Answer: The broadcast address for the 32 subnet is 63. The broadcast
address for the 64 subnet is 95. The broadcast address for the 96 subnet
is 127. The broadcast address for the 160 subnet is 191. The broadcast
address for the 192 subnet is 223 (since 224 is the mask).
 What is the valid host range of each subnet?
Answer: The valid hosts are the numbers in between the subnet and
broadcast addresses. For example, the 32 subnet valid hosts are 3362.
Let's do a third example using the Class C mask of 255.255.255.240. Here are
the answers:
 How many subnet bits are used in this mask?
Answer: 4 bits or 2^{4}2=14 subnets
 How many host bits are available per subnet?
Answer: 4 bits or 2^{4}2=14 hosts per subnet
 What are the subnet addresses?
Answer: 256240 =16, 32, 48, 64, 80, 96, 112, 128, 144. 160, 176, 192,
208 and 224 (14 subnets found by continuing to add 16 to itself.)
 What is the broadcast address of each subnet?
Answer: Here are some examples of the broadcast address: The broadcast
address for the 16 subnet is 31. The broadcast address for the 32 subnet
is 47. The broadcast address for the 64 subnet is 79. The broadcast
address for the 96 subnet is 111. The broadcast address for the 160
subnet is 175. The broadcast address for the 192 subnet is 207.
 What is the valid host range of each subnet?
Answer: The valid hosts are the numbers in between the subnet and
broadcast addresses. The 32 subnet valid hosts are 3346.
Let's do a fourth example using the Class C mask of 255.255.255.248. Here
are the answers:
 How many subnet bits are used in this mask?
Answer: 5 bits or 2^{5}2=30 subnets
 How many host bits are available per subnet?
Answer: 3 bits or 2^{3}2=6 hosts per subnet
 What are the subnet addresses?
Answer 256248 =8, 16, 24, 32, 40, 48, and so forth. The last subnet is
240 (30 subnets found by continuing to add 8 to itself).
 What is the broadcast address of each subnet?
Answer: The broadcast address for the 8 subnet is 15. The broadcast
address for the 16 subnet is 23. The broadcast address for the 48 subnet
is 55.
 What is the valid host range of each subnet?
Answer: The valid hosts are the numbers in between the subnet and
broadcast addresses. For example, the 32 subnet valid hosts are 3338.
Let's do a fifth example using the Class C mask of
255.255.255.252. Here are the answers:
 How many subnet bits are used in this mask?
Answer: 6 bits or 2^{6}2=62 subnets
 How many host bits are available per subnet?
Answer: 2 bits or 2^{2}2=2 hosts per subnet
 What are the subnet addresses?
Answer: 256252 =4, 8, 12, 16, 20, and so forth. The last subnet is 248
(62 subnets found by continuing to add 4 to itself).
 What is the broadcast address of each subnet?
Answer: The broadcast address for the 4 subnet is 7. The broadcast
address for the 8 subnet is 11. The broadcast address for the 12 subnet
is 15. The broadcast address for the 20 subnet is 23.
 What is the valid host range of each subnet?
Answer: The valid hosts are the numbers in between the subnet and
broadcast addresses. For example, the 16 subnet valid hosts are 17 and
18.
How do I use this
information?
Let's take a look at an example that will highlight how the above
information is applied.
A host configuration has an IP configuration of 192.168.10.17
255.255.255.248. What are the subnet, broadcast address, and host range that
this host is a member of? The answer is: 256248=8, 16, 24. This host is in
the 16 subnet, the broadcast address of the 16 subnet is 23, and the valid
host range is 1722. Pretty easy!
Here is an explanation of this example: First, I used 256subnetmask to get
the variable and first subnet. Then I kept adding this number to itself
until I passed the host address. The subnet is the number before the host
address, and the broadcast address is the number right before the next
subnet. The valid hosts are the numbers in between the subnet and broadcast
address.
Let's examine a second example. A host configuration has an IP configuration
of 192.168.10.37 255.255.255.240. What are the subnet, broadcast address,
and host range this host is a member of? The answer is: 256240=16, 32, 48.
This host is in the 32 subnet, the broadcast address of the 32 subnet is 47,
and the valid host range is 3346.
Let's go through a third example: A host configuration has an IP
configuration of 192.168.10.44 255.255.255.224. What are the subnet,
broadcast address, and host range this host is a member of? The answer is:
256224=32, 64. This host is in the 32 subnet, the broadcast address of the
32 subnet is 63, and the valid host range is 3362.
Here's a fourth example: A host configuration has an IP configuration of
192.168.10.17 255.255.255.252. What are the subnet, broadcast address, and
host range this host is a member of? The answer is: 256252=4, 8, 12, 16,
20. This host is in the 16 subnet, the broadcast address of the 16 subnet is
19, and the valid host range is 1718.
Let's go through a final example. A host configuration has an IP
configuration of 192.168.10.88 255.255.255.192. What are the subnet,
broadcast address and host range this host is a member of? The answer is:
256192=64, 128. This host is in the 64 subnet, the broadcast address of the
64 subnet is 128, and the valid host range must be 65126.
Conclusion
It is important to be able to subnet quickly and efficiently. After
studying the examples presented in this Daily Drill Down, you should be
familiar with this process with Class C addresses. Practice your subnetting
as much as possible, and the process will get easier and easier. In my next
Daily Drill Down, I'll take subnetting a step further and discuss subnetting
a Class B network address.

