Activity 10 – Calculating IPv4 Subnets
Objectives
Part 1: Determine IPv4 Address Subnetting
Determine the network address.
Determine the broadcast address.
Determine the number of hosts.
Part 2: Calculate IPv4 Address Subnetting
Determine the number of subnets created.
Determine number of hosts per subnet.
Determine the subnet address.
Determine the host range for the subnet.
Determine the broadcast address for the subnet.
Background / Scenario
The ability to work with IPv4 subnets and determine network and host information based on a given
IP address and subnet mask is critical to understanding how IPv4 networks operate. The first part is
designed to reinforce how to compute network IP address information from a given IP address and
subnet mask. When given an IP address and subnet mask, you will be able to determine other
information about the subnet such as:
Network address
Broadcast address
Total number of host bits
Number of hosts per subnet
In the second part of the lab, for a given IP address and subnet mask, you will determine such
information as follows:
Network address of this subnet
Broadcast address of this subnet
Range of host addresses for this subnet
Number of subnets created
Number of hosts for each subnet
Required Resources
1 PC (Windows 7, Vista, or XP with Internet access)
Optional: IPv4 address calculator
Part 1: Determine IPv4 Address Subnetting
In Part 1, you will determine the network and broadcast addresses, as well as the number of hosts,
given an IPv4 address and subnet mask.
Lab – Calculating IPv4 Subnets
REVIEW: To determine the network address, perform binary ANDing on the IPv4 address using the
subnet mask provided. The result will be the network address. Hint: If the subnet mask has decimal
value 255 in an octet, the result will ALWAYS be the original value of that octet. If the subnet mask
has decimal value 0 in an octet, the result will ALWAYS be 0 for that octet.
Example:
IP Address 192.168.10.10
Subnet Mask 255.255.255.0
==========
Result (Network) 192.168.10.0
Knowing this, you may only have to perform binary ANDing on an octet that does not have 255 or 0 in
its subnet mask portion.
Example:
IP Address 172.30.239.145
Subnet Mask 255.255.192.0
Analyzing this example, you can see that you only have to perform binary ANDing on the third octet.
The first two octets will result in 172.30 due to the subnet mask. The fourth octet will result in 0 due to
the subnet mask.
IP Address 172.30.239.145
Subnet Mask 255.255.192.0
==========
Result (Network) 172.30.?.0
Perform binary ANDing on the third octet.
Decimal Binary
239 11101111
192 11000000
=======
Result 192 11000000
Analyzing this example again produces the following result:
IP Address 172.30.239.145
Subnet Mask 255.255.192.0
==========
Result (Network) 172.30.192.0
Continuing with this example, determining the number of hosts per network can be calculated by
analyzing the subnet mask. The subnet mask will be represented in dotted decimal format, such as
255.255.192.0, or in network prefix format, such as /18. An IPv4 address always has 32 bits.
Subtracting the number of bits used for the network portion (as represented by the subnet mask)
gives you the number of bits used for hosts.
Using our example above, the subnet mask 255.255.192.0 is equivalent to /18 in prefix notation.
Subtracting 18 network bits from 32 bits results in 14 bits left for the host portion. From there, it is a
simple calculation:
2(number of host bits) - 2 = Number of hosts
Lab – Calculating IPv4 Subnets
214 = 16,384 – 2 = 16,382 hosts
Determine the network and broadcast addresses and number of host bits and hosts for the given IPv4
addresses and prefixes in the following table.
Total
IPv4 Total Number Number of
Address/Prefix Network Address Broadcast Address of Host Bits Hosts
192.168.100.25/28 192.168.100.16 192.168.100.31 4 14
172.30.10.130/30 172.30.10.128 172.30.10.131 2 2
10.1.113.75/19 10.1.96.0 10.1.127.255 13 8190
198.133.219.250/24 198.133.219.0 198.133.219.255 8 254
128.107.14.191/22 128.107.12.0 128.107.15.255 10 1022
172.16.104.99/27 172.16.104.96 172.16.104.127 5 30
Part 2: Calculate IPv4 Address Subnetting
When given an IPv4 address, the original subnet mask and the new subnet mask, you will be able to
determine:
Network address of this subnet
Broadcast address of this subnet
Range of host addresses of this subnet
Number of subnets created
Number of hosts per subnet
The following example shows a sample problem along with the solution for solving this problem:
Lab – Calculating IPv4 Subnets
Given:
Host IP Address: 172.16.77.120
Original Subnet Mask 255.255.0.0
New Subnet Mask: 255.255.240.0
Find:
Number of Subnet Bits 4
Number of Subnets Created 16
Number of Host Bits per Subnet 12
Number of Hosts per Subnet 4,094
Network Address of this Subnet 172.16.64.0
IPv4 Address of First Host on this Subnet 172.16.64.1
IPv4 Address of Last Host on this Subnet 172.16.79.254
IPv4 Broadcast Address on this Subnet 172.16.79.255
Let’s analyze how this table was completed.
The original subnet mask was 255.255.0.0 or /16. The new subnet mask is 255.255.240.0 or /20. The
resulting difference is 4 bits. Because 4 bits were borrowed, we can determine that 16 subnets were
created because 24 = 16.
The new mask of 255.255.240.0 or /20 leaves 12 bits for hosts. With 12 bits left for hosts, we use the
following formula: 212 = 4,096 – 2 = 4,094 hosts per subnet.
Binary ANDing will help you determine the subnet for this problem, which results in the network
172.16.64.0.
Finally, you need to determine the first host, last host, and broadcast address for each subnet. One
method to determine the host range is to use binary math for the host portion of the address. In our
example, the last 12 bits of the address is the host portion. The first host would have all significant
bits set to zero and the least significant bit set to 1. The last host would have all significant bits set to
1 and the least significant bit set to 0. In this example, the host portion of the address resides in the
3rd and 4th octets.
Lab – Calculating IPv4 Subnets
Description 1st Octet 2nd Octet 3rd Octet 4th Octet Description
Network/Host nnnnnnnn nnnnnnnn nnnnhhhh hhhhhhhh Subnet Mask
Binary 10101100 00010000 01000000 00000001 First Host
Decimal 172 16 64 1 First Host
Binary 10101100 00010000 01001111 11111110 Last Host
Decimal 172 16 79 254 Last Host
Binary 10101100 00010000 01001111 11111111 Broadcast
Decimal 172 16 79 255 Broadcast
Step 1: Fill out the tables below with appropriate answers given the IPv4 address,
original subnet mask, and new subnet mask.
a. Problem 1:
Given:
Host IP Address: 192.168.200.139
Original Subnet Mask 255.255.255.0
New Subnet Mask: 255.255.255.224
Find:
Number of Subnet Bits 3
Number of Subnets Created 8
Number of Host Bits per Subnet 5
Number of Hosts per Subnet 30
Network Address of this Subnet 192.168.200.128
IPv4 Address of First Host on this Subnet 192.168.200.129
IPv4 Address of Last Host on this Subnet 192.168.200.158
IPv4 Broadcast Address on this Subnet 192.168.200.159
Lab – Calculating IPv4 Subnets
b. Problem 2:
Given:
Host IP Address: 10.101.99.228
Original Subnet Mask 255.0.0.0
New Subnet Mask: 255.255.128.0
Find:
Number of Subnet Bits 9
Number of Subnets Created 512
Number of Host Bits per Subnet 15
Number of Hosts per Subnet 32,766
Network Address of this Subnet 10.101.0.0
IPv4 Address of First Host on this Subnet 10.101.0.1
IPv4 Address of Last Host on this Subnet 10.101.127.254
IPv4 Broadcast Address on this Subnet 10.101.127.255
c. Problem 3:
Given:
Host IP Address: 172.22.32.12
Original Subnet Mask 255.255.0.0
New Subnet Mask: 255.255.224.0
Find:
Number of Subnet Bits 3
Number of Subnets Created 8
Number of Host Bits per Subnet 13
Number of Hosts per Subnet 8,190
Network Address of this Subnet 172.22.32.0
IPv4 Address of First Host on this Subnet 172.22.32.1
IPv4 Address of Last Host on this Subnet 172.22.63.254
IPv4 Broadcast Address on this Subnet 172.22.63.255
Lab – Calculating IPv4 Subnets
d. Problem 4:
Given:
Host IP Address: 192.168.1.245
Original Subnet Mask 255.255.255.0
New Subnet Mask: 255.255.255.252
Find:
Number of Subnet Bits 6
Number of Subnets Created 64
Number of Host Bits per Subnet 2
Number of Hosts per Subnet 2
Network Address of this Subnet 192.168.1.244
IPv4 Address of First Host on this Subnet 192.168.1.245
IPv4 Address of Last Host on this Subnet 192.168.1.246
IPv4 Broadcast Address on this Subnet 195.168.1.247
e. Problem 5:
Given:
Host IP Address: 128.107.0.55
Original Subnet Mask 255.255.0.0
New Subnet Mask: 255.255.255.0
Find:
Number of Subnet Bits 8
Number of Subnets Created 256
Number of Host Bits per Subnet 8
Number of Hosts per Subnet 254
Network Address of this Subnet 128.107.0.0
IPv4 Address of First Host on this Subnet 128.107.0.1
IPv4 Address of Last Host on this Subnet 128.107.0.254
IPv4 Broadcast Address on this Subnet 128.107.0.255
Lab – Calculating IPv4 Subnets
f. Problem 6:
Given:
Host IP Address: 192.135.250.180
Original Subnet Mask 255.255.255.0
New Subnet Mask: 255.255.255.248
Find:
Number of Subnet Bits 5
Number of Subnets Created 32
Number of Host Bits per Subnet 3
Number of Hosts per Subnet 6
Network Address of this Subnet 192.135.250.176
IPv4 Address of First Host on this Subnet 192.135.250.177
IPv4 Address of Last Host on this Subnet 192.135.250.182
IPv4 Broadcast Address on this Subnet 192.135.250.183
Conclusion
Why is the subnet mask so important when analyzing an IPv4 address?
Grasping how subnet masks work is crucial when dealing with IP
addresses, as they define how devices are grouped within a
network. By learning to compute details like network and
broadcast addresses, as well as available hosts, we gain better
insight into how networks operate and can be customized.
Subnetting goes beyond being a technical task—it's a core skill
for creating structured and efficient networks.
Lab – Calculating IPv4 Subnets
Reflections:
Learning to calculate subnets was challenging at first, but with
patience and practice, I came to understand it as a logical
puzzle. The activity deepened my appreciation for the planning
involved in networking.
Submitted by:
Name: PAULAR, MANUEL ALDRIN B.
Section: BSECE-3B
Date: May 5, 2025
