University of Babylon/College of Information Technology/ Information Network Dept. /
First Class / Second Semester/ Subject : Network and Distributed System/ Lecture : 2
Type of addressing in IPv4
Within the address range of each IPv4 network, we have three types of addresses:
Network address - The address by which we refer to the network
Broadcast address - A special address used to send data to all hosts in the network
Host addresses - The addresses assigned to the end devices in the network
Network Prefixes
An important question is: How do we know how many bits represent the network
portion and how many bits represent the host portion? When we express an IPv4 network
address, we add a prefix length to the network address. The prefix length is the number of
bits in the address that gives us the network portion. For example, in 172.16.4.0 /24, the /24
is the prefix length - it tells us that the first 24 bits are the network address. This leaves the
remaining 8 bits, the last octet, as the host portion. Later in this chapter, we will learn more
about another entity that is used to specify the network portion of an IPv4 address to the
network devices. It is called the subnet mask.
The subnet mask consists of 32 bits, just as the address does, and uses 1s and 0s to indicate which
bits of the address are network bits and which bits are hosts bits.
Networks are not always assigned a /24 prefix. Depending on the number of hosts on
the network, the prefix assigned may be different. Having a different prefix number changes
the host range and broadcast address for each network.
Notice that the network address could remain the same, but the host range and the
broadcast address are different for the different prefix lengths. In this figure you can also
see that the number of hosts that can be addressed on the network changes as well.
Lecturer : Ahmed M. Al-Saleh & Mouayad Najim
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University of Babylon/College of Information Technology/ Information Network Dept. /
First Class / Second Semester/ Subject : Network and Distributed System/ Lecture : 2
Understanding Subnetting
Subnetting allows you to create multiple logical networks that exist within a single Class
A, B, or C network. If you do not subnet, you will only be able to use one network from your Class
A, B, or C network, which is unrealistic.
Formula for calculating subnets
Use this formula to calculate the number of subnets:
2^n where n = the number of bits borrowed
The number of hosts
To calculate the number of hosts per network, we use the formula of 2^n - 2
where n = the number of bits left for hosts.
Each data link on a network must have a unique network ID, with every node on that link
being a member of the same network. If you break a major network (Class A, B, or C) into smaller
subnetworks, it allows you to create a network of interconnecting subnetworks. Each data link on
this network would then have a unique network/subnetwork ID. Any device, or gateway,
connecting n networks/subnetworks has n distinct IP addresses, one for each network / subnetwork
that it interconnects.
Lecturer : Ahmed M. Al-Saleh & Mouayad Najim
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University of Babylon/College of Information Technology/ Information Network Dept. /
First Class / Second Semester/ Subject : Network and Distributed System/ Lecture : 2
To subnet a network, extend the natural mask using some of the bits from the host ID
portion of the address to create a subnetwork ID. For example, given a Class C network of
204.15.5.0 which has a natural mask of 255.255.255.0, you can create subnets in this manner:
204.15.5.0 -
11001100.00001111.00000101.00000000
255.255.255.224 - 11111111.11111111.11111111.11100000
------------------------------------------------|sub|---By extending the mask to be 255.255.255.224, you have taken three bits (indicated by "sub") from
the original host portion of the address and used them to make subnets. With these three bits, it is
possible to create eight subnets. With the remaining five host ID bits, each subnet can have up to
32 host addresses, 30 of which can actually be assigned to a device since host ids of all zeros or
all ones are not allowed (it is very important to remember this). So, with this in mind, these subnets
have been created.
204.15.5.0 255.255.255.224
host address range 1 to 30
204.15.5.32 255.255.255.224
host address range 33 to 62
204.15.5.64 255.255.255.224
host address range 65 to 94
204.15.5.96 255.255.255.224
host address range 97 to 126
204.15.5.128 255.255.255.224 host address range 129 to 158
204.15.5.160 255.255.255.224 host address range 161 to 190
204.15.5.192 255.255.255.224 host address range 193 to 222
204.15.5.224 255.255.255.224 host address range 225 to 254
Note: There are two ways to denote these masks. First, since you are using three bits more than
the "natural" Class C mask, you can denote these addresses as having a 3-bit subnet mask. Or,
secondly, the mask of 255.255.255.224 can also be denoted as /27 as there are 27 bits that are set
in the mask. This second method is used with CIDR. Using this method, one of thse networks can
be described with the notation prefix/length. For example, 204.15.5.32/27 denotes the network
204.15.5.32 255.255.255.224. When appropriate the prefix/length notation is used to denote the
mask throughout the rest of this document.
Lecturer : Ahmed M. Al-Saleh & Mouayad Najim
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University of Babylon/College of Information Technology/ Information Network Dept. /
First Class / Second Semester/ Subject : Network and Distributed System/ Lecture : 2
The network subnetting scheme in this section allows for eight subnets, and the network might
appear as:
Figure 2
Notice that each of the routers in Figure 2 is attached to four subnetworks, one subnetwork
is common to both routers. Also, each router has an IP address for each subnetwork to which it is
attached. Each subnetwork could potentially support up to 30 host addresses.
This brings up an interesting point. The more host bits you use for a subnet mask, the more
subnets you have available. However, the more subnets available, the less host addresses available
per subnet. For example, a Class C network of 204.17.5.0 and a mask of 255.255.255.224 (/27)
allows you to have eight subnets, each with 32 host addresses (30 of which could be assigned to
devices). If you use a mask of 255.255.255.240 (/28), the break down is:
204.15.5.0 -
11001100.00001111.00000101.00000000
255.255.255.240 - 11111111.11111111.11111111.11110000
--------------------------------------------------|sub |---
Since you now have four bits to make subnets with, you only have four bits left for host addresses.
So in this case you can have up to 16 subnets, each of which can have up to 16 host addresses (14
of which can be assigned to devices).
Take a look at how a Class B network might be subnetted. If you have network 172.16.0.0 ,then
you know that its natural mask is 255.255.0.0 or 172.16.0.0/16. Extending the mask to anything
Lecturer : Ahmed M. Al-Saleh & Mouayad Najim
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University of Babylon/College of Information Technology/ Information Network Dept. /
First Class / Second Semester/ Subject : Network and Distributed System/ Lecture : 2
beyond 255.255.0.0 means you are subnetting. You can quickly see that you have the ability to
create a lot more subnets than with the Class C network. If you use a mask of 255.255.248.0 (/21),
how many subnets and hosts per subnet does this allow for?
172.16.0.0 -
10101100.00010000.00000000.00000000
255.255.248.0 - 11111111.11111111.11111000.00000000
---------------------------| sub |-------------------
You are using five bits from the original host bits for subnets. This will allow you to have 32
subnets (25). After using the five bits for subnetting, you are left with 11 bits for host addresses.
This will allow each subnet so have 2048 host addresses (211), 2046 of which could be assigned to
devices.
Note: In the past, there were limitations to the use of a subnet 0 (all subnet bits are set to zero) and
all ones subnet (all subnet bits set to one). Some devices would not allow the use of these subnets.
Cisco Systems devices will allow the use of these subnets when the ip subnet zero command is
configured.
Examples
Sample Exercise 1
Now that you have an understanding of subnetting, put this knowledge to use. In this example, you
are given two address / mask combinations, written with the prefix/length notation, which have
been assigned to two devices. Your task is to determine if these devices are on the same subnet or
different subnets. You can do this by using the address and mask of each device to determine to
which subnet each address belongs.
DeviceA: 172.16.17.30/20
DeviceB: 172.16.28.15/20
Lecturer : Ahmed M. Al-Saleh & Mouayad Najim
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University of Babylon/College of Information Technology/ Information Network Dept. /
First Class / Second Semester/ Subject : Network and Distributed System/ Lecture : 2
Determining the Subnet for DeviceA:
172.16.17.30 - 10101100.00010000.00010001.00011110
255.255.240.0 - 11111111.11111111.11110000.00000000
---------------------------| sub|--------------------subnet =
10101100.00010000.00010000.00000000 = 172.16.16.0
Looking at the address bits that have a corresponding mask bit set to one, and setting all the other
address bits to zero (this is equivalent to performing a logical "AND" between the mask and
address), shows you to which subnet this address belongs. In this case, DeviceA belongs to subnet
172.16.16.0.
Determining the Subnet for DeviceB:
172.16.28.15 - 10101100.00010000.00011100.00001111
255.255.240.0 - 11111111.11111111.11110000.00000000
---------------------------| sub|-----------subnet =
10101100.00010000.00010000.00000000 = 172.16.16.0
From these determinations, DeviceA and DeviceB have addresses that are part of the same subnet.
Subnetting
The procedure in which we browse some Host bits from the Host portion and add it in
Network bits in Network Portion this procedure is called Subnetting. In Subnetting we increase the
Network Portion and decrease the Host Portion.
It means as Network Portion increases option of Sub networks also increases but the no of Hosts in each
network start to decrease.
For Example:
124.192.135.159 as we know an IP address is of 32 Bits





It Is an IP address of Class A (Because in First Octet Value range is 0 – 126)
Its Network Portion is of 8 bits
Its Host Portion is of 24 bits
Its Subnet mask is 255.0.0.0
Its Network ID is 124.0.0.0
Lecturer : Ahmed M. Al-Saleh & Mouayad Najim
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University of Babylon/College of Information Technology/ Information Network Dept. /
First Class / Second Semester/ Subject : Network and Distributed System/ Lecture : 2
Note:
IP addresses are also represented as (IP/Network Bits)
Class A

124.192.135.159/8

In Class A Network Portion is of 8 Bits
Class B

189.200.191.239/16

In Class B Network Portion is of 16 Bits
Class C

193.220.164.223/24

In Class C Network Portion is of 24 Bits
200.100.100.0/24
(IP of Class C + Network Portion = 24 Bits + Host Portion = 8 Bits)
Let us Borrow 2-Bits from Host Portion (which have Host Portion = 8-Bits)
Now Host Portion have Total Bits = 6
Add it in Network Portion (which have Network Portion = 24-Bits)
Now Network Portion have Total Bits = 26
What is Custom Sub net Mask
After Subnetting Default Subnet Mask of the IP address becomes Custom Subnet Mask.
How many Subnets can form when we borrow some Bits from Host portion and add it in Network Portion?
We can produce 2n (n = is equal to number of bits Borrow from Host Portion)
If we Borrow 1 Bit 
2nn = 1
21
2  Subnets can form
If we Borrow 2 Bits
2nn = 2
22
4  Subnets can form
If we Borrow 3 Bits
2nn = 3
23
8  Subnets can form
If we Borrow 4 Bits
2nn = 4
24
16 Subnets can form
If we Borrow 5 Bits
2nn = 5
25
32 Subnets can form
If we Borrow 6 Bits
2nn = 6
26
64 Subnets can form
Lecturer : Ahmed M. Al-Saleh & Mouayad Najim
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University of Babylon/College of Information Technology/ Information Network Dept. /
First Class / Second Semester/ Subject : Network and Distributed System/ Lecture : 2
How may Maximum Bits be able to borrow from Host Portion?
We can borrow maximum 6 bits from Host Portion
Note:
If number of subnets increases congestion in the routing tables would be happened because size of routing
tables increases but if Bandwidth is sufficient no congestion will take place.
Types of Subnetting
There are two types of Subnetting
1
Fixed Length Subnetting
or
Fixed Length Subnet Masking
2
Variable Length Subnetting
or
Variable Length Subnet Masking
1
Fixed Length Subnet Masking
If we are sure that we have limit of subnets and not increases from the limit it is called Fixed
Length Subnetting
2
Variable Length Subnet Masking
---------------------------------------------------------------------------------------------------------------------------------------
Question: How Many Subnets can form using following IP Address we do sub
netting of 2-Bits and How many Hosts Can be in each subnet?
200.100.100.0
Answer:
Given IP address is 200.100.100.0
1
Class

C
2
Network Portion

24-Bits
3
Host Portion

08-Bits
4
Representation of IP

200.100.100.0/24
Lecturer : Ahmed M. Al-Saleh & Mouayad Najim
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University of Babylon/College of Information Technology/ Information Network Dept. /
First Class / Second Semester/ Subject : Network and Distributed System/ Lecture : 2

5
Subnetting
6
After Subnetting Network Portion 
7
After Subnetting Host Portion

06-Bits
8
After Subnetting IP

200.100.100.0/26
9
Number of Subnets
10
Number of Hosts per Subnet

2-Bits
26-Bits
2n
n=2 22

4
2n
n=6 26

64
Default Subnet Mask
Default
Subnet Mask
of
Class Full IP Address 200.100.100.0/24
As we know to find out the Default Subnet Mask we ON all 24-Bits of Network Portion of
Class Full IP address
200
.
100
11111111.
255
.
.
100 .
11111111.
255
.
Network ID
0/24
1111111.
255 .
of
0
0
Class Full IP
200.100.100.0/24
200.100.100.0
Custom Subnet Mask
Custom Subnet Mask of Class Less IP Address 200.100.100.0/26
As we Know to find out the Custom Subnet Mask we ON all 26-Bits of Network Portion of
class Less IP Address
200
.
100
11111111.
255
.
.
100
11111111.
255
.
255
.
0/26
11111111.
.
11000000
192
Lecturer : Ahmed M. Al-Saleh & Mouayad Najim
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University of Babylon/College of Information Technology/ Information Network Dept. /
First Class / Second Semester/ Subject : Network and Distributed System/ Lecture : 2
Subnet ID of Given
Class Less IP
200
.
100
.
100
.
65
200
.
100
.
100
.
01000001
255
.
255
.
255
.
11000000
200
.
100
.
100
.
01000000
200
.
100
.
100
.
64
200.100.100.0/26
200.100.100.65/26
Custom Subnet Mask
200.100.100.65
100.100.100.01000001
255.255.255.11000000
255.255.255.192
100.100.100.01000000
200.100.100.64
Table 1 : Summery of Subnetting for Class A,B And C)
# bits
Subnet Mask
CIDR
# Subnets
# Hosts
1
255.128.0.0
/9
2
8388608
2
3
4
255.192.0.0
255.224.0.0
255.240.0.0
/10
/11
/12
4
8
16
4194304
2097152
1048576
5
6
7
8
9
10
11
12
255.248.0.0
255.252.0.0
255.254.0.0
255.255.0.0
255.255.128.0
255.255.192.0
255.255.224.0
255.255.240.0
/13
/14
/15
/16
/17
/18
/19
/20
32
64
128
256
512
1024
2048
4096
524288
262144
131072
65536
32768
16384
8192
4096
13
14
255.255.248.0
255.255.252.0
/21
/22
8192
16384
2048
1024
Lecturer : Ahmed M. Al-Saleh & Mouayad Najim
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University of Babylon/College of Information Technology/ Information Network Dept. /
First Class / Second Semester/ Subject : Network and Distributed System/ Lecture : 2
15
16
17
18
19
20
21
22
255.255.254.0
255.255.255.0
255.255.255.128
255.255.255.192
255.255.255.224
255.255.255.240
255.255.255.248
255.255.255.252
/23
/24
/25
/26
/27
/28
/29
/30
32768
65536
131072
262144
524288
1048576
2097152
4194304
512
256
128
64
32
16
8
4
23
255.255.255.254
/31
8388608
2
# bits
Subnet Mask
CIDR
# Subnets
# Hosts
1
255.255.128.0
/17
2
32768
2
3
4
5
6
255.255.192.0
255.255.224.0
255.255.240.0
255.255.248.0
255.255.252.0
/18
/19
/20
/21
/22
4
8
16
32
64
16384
8192
4096
2048
1024
7
8
9
10
11
12
13
14
255.255.254.0
255.255.255.0
255.255.255.128
255.255.255.192
255.255.255.224
255.255.255.240
255.255.255.248
255.255.255.252
/23
/24
/25
/26
/27
/28
/29
/30
128
256
512
1024
2048
4096
8192
16384
512
256
128
64
32
16
8
4
15
255.255.255.254
/31
32768
2
# bits
Subnet Mask
CIDR
# Subnets
# Hosts
1
255.255.255.128
/25
2
128
2
3
4
5
6
255.255.255.192
255.255.255.224
255.255.255.240
255.255.255.248
255.255.255.252
/26
/27
/28
/29
/30
4
8
16
32
64
64
32
16
8
4
7
255.255.255.254
/31
128
2
Lecturer : Ahmed M. Al-Saleh & Mouayad Najim
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University of Babylon/College of Information Technology/ Information Network Dept. /
First Class / Second Semester/ Subject : Network and Distributed System/ Lecture : 2
Subnetting (Dividing Networks into Right Sizes)
Every network within the internetwork of a corporation or organization is designed
to accommodate a finite number of hosts.
Some networks, such as point-to-point WAN links, only require a maximum of two hosts. Other
networks, such as a user LAN in a large building or department, may need to accommodate
hundreds of hosts. Network administrators need to devise the internetwork addressing scheme to
accommodate the maximum number of hosts for each network. The number of hosts in each
division should allow for growth in the number of hosts.
Determine the Total Number of Hosts
First, consider the total number of hosts required by the entire corporate internetwork.
We must use a block of addresses that is large enough to accommodate all devices in all the
corporate networks. This includes end user devices, servers, intermediate devices, and router
interfaces.
See Step 1 of the figure.
Lecturer : Ahmed M. Al-Saleh & Mouayad Najim
Page 25
University of Babylon/College of Information Technology/ Information Network Dept. /
First Class / Second Semester/ Subject : Network and Distributed System/ Lecture : 2
Consider the example of a corporate internetwork that needs to accommodate 800
hosts in its four locations.
Determine the Number and Size of the Networks
Next, consider the number of networks and the size of each required based on
common groupings of hosts.
See Step 2 of the figure.
We subnet our network to overcome issues with location, size, and control. In
designing the addressing, we consider the factors for grouping the hosts that we
discussed previously:
 Grouping based on common geographic location
 Grouping hosts used for specific purposes
 Grouping based on ownership
Lecturer : Ahmed M. Al-Saleh & Mouayad Najim
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University of Babylon/College of Information Technology/ Information Network Dept. /
First Class / Second Semester/ Subject : Network and Distributed System/ Lecture : 2
Each WAN link is a network. We create subnets for the WAN that interconnect
different geographic locations. When connecting the different locations, we use a
router to account for the hardware differences between the LANs and the WAN.
Although hosts in a common geographic location typically comprise a single block
of addresses, we may need to subnet this block to form additional networks at each
location. We need to create subnetworks at the different locations that have hosts for
common user needs. We may also have other groups of users that require many
network resources, or we may have many users that require their own subnetwork.
Additionally, we may have subnetworks for special hosts such as servers. Each of
these factors needs to be considered in the network count.
We also have to consider any special security or administrative ownership needs that
require additional networks.
One useful tool in this address planning process is a network diagram. A
diagram allows us to see the networks and make a more accurate count.
To accommodate 800 hosts in the company's four locations, we use binary arithmetic
to allocate a /22 block (2^10-2=1022).
Allocating Addresses
Now that we have a count of the networks and the number of hosts for each network,
we need to start allocating addresses from our overall block of addresses.
See Step 3 of the figure.
Lecturer : Ahmed M. Al-Saleh & Mouayad Najim
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University of Babylon/College of Information Technology/ Information Network Dept. /
First Class / Second Semester/ Subject : Network and Distributed System/ Lecture : 2
This process begins by allocating network addresses for locations of special
networks. We start with the locations that require the most hosts and work down to
the point-to-point links. This process ensures that large enough blocks of addresses
are made available to accommodate the hosts and networks for these locations.
When making the divisions and assignment of available subnets, make sure that there
are adequately-sized address blocks available for the larger demands. Also, plan carefully to
ensure that the address blocks assigned to the subnet do not overlap.
Lecturer : Ahmed M. Al-Saleh & Mouayad Najim
Page 28
University of Babylon/College of Information Technology/ Information Network Dept. /
First Class / Second Semester/ Subject : Network and Distributed System/ Lecture : 2
In our example, we now allocate blocks of addresses to the four locations as well as the WAN
links.
With the major blocks allocated, next we subnet any of the locations that require
dividing. In our example, we divide the corporate HQ into two networks.
Lecturer : Ahmed M. Al-Saleh & Mouayad Najim
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University of Babylon/College of Information Technology/ Information Network Dept. /
First Class / Second Semester/ Subject : Network and Distributed System/ Lecture : 2
we will look at addressing from another view. We will consider subnetting based on
the number of hosts, including router interfaces and WAN connections. This
scenario has the following requirements:
• AtlantaHQ 58 host addresses
• PerthHQ 26 host addresses
• SydneyHQ 10 host addresses
• CorpusHQ 10 host addresses
• WAN links 2 host addresses (each)
When creating an appropriate addressing scheme, always begin with the largest
requirement. In this case, the AtlantaHQ, with 58 users, has the largest requirement.
Starting with 192.168.15.0, we will need 6 host bits to accommodate the requirement
of 58 hosts, this allows 2 additional bits for the network portion. The prefix for this
network would be /26 and a subnet mask of 255.255.255.192.
Let's begin by subnetting the original address block of 192.168.15.0 /24. Using the
Usable hosts = 2^n - 2 formula, we calculate that 6 host bits allow 62 hosts in the
subnet. The 62 hosts would meet the required 58 hosts of the AtlantaHQ company
router.
Address: 192.168.15.0
In Binary: 11000000.10101000.00001111.00000000
Mask: 255.255.255.192/26 Bits in binary : 11111111.11111111.11111111.11000000
Lecturer : Ahmed M. Al-Saleh & Mouayad Najim
Page 30
University of Babylon/College of Information Technology/ Information Network Dept. /
First Class / Second Semester/ Subject : Network and Distributed System/ Lecture : 2
This tables shows the the Networks IP Addressing for each Corporation after using (VLSM) depeond on
the number of Hosts.
Lecturer : Ahmed M. Al-Saleh & Mouayad Najim
Page 31