Republic of the Philippines
Cagayan State University
www.csu.edu.ph
IT-224 Network Management 2
Lecture Activity 2.7
Title: IPv4 Fixed Length Subnet Mask (FLSM) using CIDR Formulas
Objectives:
1. To demonstrate the FLSM step by step process using Rule-based CIDR Formulas
2. To solve different FLSM problems with any valid requirements
Reading Materials
FLSM (Fixed Length Subnet Mask) using CIDR Formulas and Tables
Now you’re ready to move on to a more complicated example, performing calculations using
formulas, without so much binary involved. Suppose you want to divide your local network, which
has a network ID of 192.168.89.0, into six (6) subnets to correspond to your building’s six floors.
The following steps walk you through the process:
1. Decide how many bits to borrow—How many bits must you borrow from the host portion of the
IP addresses in order to get six subnets? Use this formula to determine the number of bits:
2n = Y
n equals the number of bits that must be switched from the host address to the network ID.
Y equals the number of subnets that result. Because you want six separate subnets (meaning that
Y, in this case, is 6), the equation becomes 2n = 6.
Experiment with different values for n until you find a value large enough to give you at least the
number of subnets you need. For example, you know that 22 = 4 ; however, 4 is not high enough.
Instead consider that 23 = 8 ; this will give you enough subnets to meet your current needs and
allow room for future growth. Now that n equals 3, you know that three bits in the host addresses
of your Class C network must change to network ID bits. You also know that three bits in your
subnet mask must change from 0 to 1.
2. Determine the subnet mask—As you know, the default subnet mask for a Class C network is
255.255.255.0, or 11111111 11111111 11111111 00000000. In this default subnet mask, the first
24 bits indicate the position of network information. Changing three of the default subnet mask’s
bits from host to network information gives you the subnet mask 11111111 11111111 11111111
11100000. In this modified subnet mask, the first 27 bits indicate the bits for the network ID. Note
that for this Class C network whose network ID is 192.168.89.0, the slash notation would now be
192.168.89.0/27 because 27 bits of the subnets’ addresses are used to provide network
information. Converting from binary to the more familiar dotted decimal notation, this subnet
mask becomes 255.255.255.224. When you configure the TCP/IP properties of clients on your
network, as shown in Figure 2.7-3, you would specify this new subnet mask.
PAGE 1 Authors : Fernando M. Jaminola Jr., MIT, Marvin Q. Uanang, MIT and Emerito A. Lanojan, MIT
No part of this document can be revised, published or shared without the prior written consent of the authors.
Edited : March 21,2021.
Republic of the Philippines
Cagayan State University
www.csu.edu.ph
Figure 2.7-1 TCP/IPv4 Client Configuration for a subnet in Ubuntu Desktop
3. Calculate the network ID for each subnet—The first three octets of the network ID for the Class
C network 192.168.89.0 is the same for all eight possible subnets. The network IDs differ in the last
octet. Use the magic number to calculate them as follows:
• Subnet 1 Network ID: 192.168.89.0
• Subnet 2 Network ID: 192.168.89.0 1 32 yields 192.168.89.32
• Subnet 3 Network ID: 192.168.89.32 1 32 yields 192.168.89.64
• Subnet 4 Network ID: 192.168.89.64 1 32 yields 192.168.89.96
• Subnet 5 Network ID: 192.168.89.96 1 32 yields 192.168.89.128
• Subnet 6 Network ID: 192.168.89.128 1 32 yields 192.168.89.160
• Subnet 7 Network ID: 192.168.89.160 1 32 yields 192.168.89.192
• Subnet 8 Network ID: 192.168.89.192 1 32 yields 192.168.89.224
This method of adding on the same number over and over is called skip-counting
4. Determine the IP address range for each subnet—Recall that you have borrowed 3 bits from
what used to be host information in the IP address. That leaves 5 bits instead of 8 available in the
last octet of your Class C addresses to identify hosts. To calculate the number of possible hosts,
keep in mind that each of the 5 bits has two possibilities, a 1 or a 0. Therefore, the number of host
addresses is 2 x 2 x 2 x 2 x 2, or 32 host addresses. But you can’t use two of these addresses for
hosts because one is used for the network ID (the one where all five bits are 0 in binary) and one
for the broadcast address (the one where all five bits are 1 in binary). That leaves you 30 host
addresses in each subnet
As a shortcut to calculating the number of hosts, you can use the formula:
2h – 2 = Z
• h equals the number of bits remaining in the host portion.
• Z equals the number of hosts available in each subnet.
So, 25- 2 is 32 -2 yields 30 possible hosts per subnet.
In this example, you can have a maximum of 8 (number of subnets) at 30 (number of hosts per
subnet), or 240, unique host addresses on the entire, larger network. Each time you subnet a
network, you lose two possible host addresses with each subnet. This overhead is the price you
PAGE 2 Authors : Fernando M. Jaminola Jr., MIT, Marvin Q. Uanang, MIT and Emerito A. Lanojan, MIT
No part of this document can be revised, published or shared without the prior written consent of the authors.
Edited : March 21,2021.
Republic of the Philippines
Cagayan State University
www.csu.edu.ph
pay for subnetting a network, in exchange for the advantages you gain. Once you know the
network ID of the subnets, calculating the address range of hosts in a subnet is easy. For example,
take subnet 5. The network ID is 192.168.89.128. Because you won’t use the network ID for a host
address, you start with the next value and keep going until you reach the broadcast address for
the subnet, yielding for this particular subnet a total of 30 addresses. Therefore, the address range
for subnet 5 is 192.168.89.129 through 192.168.89.158. (The last value 158 is 128 + 30.)
Table 2.7-1 lists the network ID, broadcast address, and the range of usable host addresses for
each of the eight subnets in this sample network. Together, the existing network ID plus the
additional bits used for subnet information are sometimes called the extended network prefix.
Table 2.7-1 List of network ID, Broadcast Address and the range of usable address
for each eight (8) subnets in the Network
PAGE 3 Authors : Fernando M. Jaminola Jr., MIT, Marvin Q. Uanang, MIT and Emerito A. Lanojan, MIT
No part of this document can be revised, published or shared without the prior written consent of the authors.
Edited : March 21,2021.
Republic of the Philippines
Cagayan State University
www.csu.edu.ph
Activities
1. Explain briefly the process of FLSM using CIDR Rule-based Formulas?
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2. You have an Office Building with 4 Floors. As Chief Technology Officer you want to
Segment the Office with assigned IP Address of 192.168.89.0, into four (4) subnets.
Fill up your answers in Table 2.7-2 List of network ID, Broadcast Address and the range of
usable address as required.
Table 2.7-2 List of Subnet number, Network ID, Range of usable address and Broadcast Address
References
1. West, et al., 2019. Network Guide to Networks 8th Edition, pages 478-481 of 881
2. Youtube Tutorial for FLSM Subnetting by Sunny (792) subnetting is simple - YouTube
PAGE 4 Authors : Fernando M. Jaminola Jr., MIT, Marvin Q. Uanang, MIT and Emerito A. Lanojan, MIT
No part of this document can be revised, published or shared without the prior written consent of the authors.
Edited : March 21,2021.