# Fixed Length Subnet Masking (FLSM) in Computer Networks

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## I. Introduction

Hey there, fellow networkers! In this article, we’ll be taking a deep dive into Fixed Length Subnet Masking (FLSM). But before we get into the nitty-gritty details, let’s start with some basic definitions.

### Explanation of Subnetting

Subnetting is the process of dividing a large network into smaller subnetworks, or subnets. This is done by borrowing bits from the host portion of an IP address and using them to create a new subnet mask. By doing this, we can create multiple smaller networks that are easier to manage and more efficient.

### Purpose of Subnetting

So why do we subnet in the first place? Well, there are several reasons. First and foremost, subnetting allows us to better manage our network resources. By dividing a large network into smaller subnets, we can more easily control network traffic and reduce congestion. Subnetting also allows us to implement better security measures by segmenting our network and isolating sensitive information.

### Overview of Fixed Length Subnet Masking (FLSM)

Now that we have a basic understanding of subnetting, let’s talk about Fixed Length Subnet Masking (FLSM). FLSM is a subnetting method where each subnet has the same number of hosts. In other words, we use a fixed number of bits to represent the network portion of the IP address and a fixed number of bits to represent the host portion. This means that each subnet has a fixed number of hosts, and we can easily calculate how many subnets we can create based on the number of bits we borrow from the host portion of the IP address.

## II. IP Addressing and Binary Representation

Now that we understand the basics of subnetting and FLSM, let’s dive a bit deeper into IP addressing and how it relates to subnetting.

An IP address is a unique identifier assigned to every device on a network. It consists of four sets of numbers, each ranging from 0 to 255, separated by periods. For example, 192.168.0.1 is a common IP address used for devices on a local network.

IP addresses can be either IPv4 or IPv6. IPv4 is the most commonly used addressing scheme and uses 32 bits to represent an IP address. This means there are 2^32 possible unique IP addresses. However, not all of these addresses are available for use, as some are reserved for special purposes such as multicast and private networks.

IPv6, on the other hand, uses 128 bits to represent an IP address, which provides significantly more unique addresses. However, due to the slower adoption of IPv6 and the need to support legacy systems, IPv4 is still widely used.

### Binary Representation of IP Addresses

While IP addresses are typically represented in decimal format, they are actually stored and processed in binary format. This means that each of the four sets of numbers in an IP address is actually represented by 8 bits, or one byte.

For example, the IP address 192.168.0.1 can be represented in binary as:

11000000.10101000.00000000.00000001

This binary representation is important when subnetting, as it allows us to easily identify the network portion and the host portion of an IP address.

In the early days of the internet, IP addresses were assigned using a classful addressing scheme. This meant that IP addresses were divided into classes based on the number of bits used to represent the network portion of the address.

Class A addresses, for example, used the first octet to represent the network portion of the address, with the remaining three octets representing the host portion. This meant that Class A addresses could support a large number of hosts, but only a limited number of networks.

Class B addresses, on the other hand, used the first two octets to represent the network portion of the address, with the remaining two octets representing the host portion. This allowed for more networks, but fewer hosts per network.

Class C addresses used the first three octets to represent the network portion of the address, with the remaining octet representing the host portion. This allowed for even more networks, but fewer hosts per network.

Classful addressing was later replaced by Classless Inter-Domain Routing (CIDR), which allowed for more flexible subnetting and better use of available IP address space. However, it’s still important to understand classful addressing when working with legacy systems.

## III. Subnetting

In the previous sections, we’ve covered the basics of subnetting and IP addressing. Now, let’s take a closer look at subnetting and how it works.

### Benefits of Subnetting

Subnetting offers several benefits, including:

• Efficient use of IP addresses: By dividing a large network into smaller subnets, we can more efficiently use the available IP address space.
• Better network performance: By reducing the size of broadcast domains, we can reduce network congestion and improve network performance.
• Improved network security: By segmenting the network into smaller subnets, we can implement more granular security policies and isolate sensitive data.

### How Subnetting Works

Subnetting works by borrowing bits from the host portion of an IP address and using them to create a new subnet mask. The subnet mask is used to determine the network and host portions of an IP address.

For example, let’s say we have the IP address 192.168.0.1 and we want to divide our network into two subnets. To do this, we need to borrow a bit from the host portion of the address to create a subnet mask of 255.255.255.128. This means that the first 25 bits of the IP address represent the network portion, and the remaining 7 bits represent the host portion.

The two subnets we create would have the following network addresses:

• Subnet 1: 192.168.0.0/25
• Subnet 2: 192.168.0.128/25

Each subnet can then be further divided into smaller subnets as needed by borrowing additional bits from the host portion of the address.

### Terms Used in Subnetting

When working with subnetting, there are several terms you should be familiar with:

• Subnet mask: The subnet mask is used to determine the network and host portions of an IP address.
• CIDR notation: CIDR notation is a shorthand way of representing the subnet mask. It consists of the network address followed by a slash (/) and the number of bits in the subnet mask. For example, 192.168.0.0/24 represents a subnet with a subnet mask of 255.255.255.0.

### Types of Subnetting

There are several types of subnetting, including:

• Fixed Length Subnet Masking (FLSM): FLSM, which we discussed earlier, is a subnetting method where each subnet has the same number of hosts.
• Variable Length Subnet Masking (VLSM): VLSM is a subnetting method where subnets can have different numbers of hosts. This allows for more efficient use of available IP address space.
• Classless Inter-Domain Routing (CIDR): CIDR is a subnetting method that allows for more flexible subnetting by using variable-length subnet masks. This allows for more efficient use of available IP address space and better routing.

Understanding subnetting and its various methods is crucial for network administrators and engineers. By effectively dividing networks into subnets, we can improve network performance, security, and efficiency.

## IV. Fixed Length Subnet Masking (FLSM)

### Definition and Explanation of FLSM

Fixed Length Subnet Masking (FLSM) is a subnetting method where each subnet has the same number of hosts. This means that the subnet mask is the same for all subnets, and the number of hosts in each subnet is fixed.

For example, if we have a Class C IP address with a default subnet mask of 255.255.255.0, we can divide it into 8 subnets of 30 hosts each by using a subnet mask of 255.255.255.224.

FLSM is a simple and straightforward method of subnetting that is easy to implement and manage. However, it can lead to inefficient use of IP address space, especially when subnets require different numbers of hosts.

• Simplicity: FLSM is a simple and straightforward method of subnetting that is easy to understand and implement.
• Predictability: Because each subnet has the same number of hosts, it is easy to predict the size of each subnet and how many subnets can be created.
• Manageability: Because the subnet mask is the same for all subnets, it is easy to manage and troubleshoot the network.

• Inefficient use of IP address space: FLSM can lead to inefficient use of IP address space, especially when subnets require different numbers of hosts.
• Limited scalability: FLSM is not very scalable and may not be suitable for large and complex networks.
• Wasted IP addresses: FLSM can lead to wasted IP addresses, especially when subnets require fewer hosts than the fixed number of hosts allocated to each subnet.

### How to Calculate Subnets using FLSM

Calculating subnets using FLSM is a simple process that involves the following steps:

1. Determine the number of hosts required for each subnet.
2. Choose a subnet mask that accommodates the required number of hosts.
3. Determine the number of subnets required.
4. Allocate IP addresses to each subnet.

Let’s look at an example to see how this works:

Suppose we have the Class C IP address 192.168.10.0 and we want to divide it into 6 subnets, each with 10 hosts.

1. Determine the number of hosts required for each subnet: 10 hosts per subnet.
2. Choose a subnet mask that accommodates the required number of hosts: We need a subnet mask that allows for at least 10 hosts per subnet. The subnet mask 255.255.255.240 allows for 16 IP addresses per subnet, of which 14 can be used for hosts.
3. Determine the number of subnets required: We need 6 subnets.
4. Allocate IP addresses to each subnet:
Subnet 1: 192.168.10.0/28
Subnet 2: 192.168.10.16/28
Subnet 3: 192.168.10.32/28
Subnet 4: 192.168.10.48/28
Subnet 5: 192.168.10.64/28
Subnet 6: 192.168.10.80/28

In this example, we used a subnet mask of 255.255.255.240 to create 6 subnets of 14 hosts each.

FLSM is a useful subnetting method that is easy to understand and implement. However, it may not be suitable for all network configurations and may lead to inefficient use of IP address space.

## V. FLSM Examples

In this section, we will provide examples of subnetting Class C, Class B, and Class A IP addresses using FLSM.

### Example 1: Subnetting a Class C Address using FLSM

Suppose we have the Class C IP address 192.168.10.0 and we want to divide it into 4 subnets, each with 14 hosts.

1. Determine the number of hosts required for each subnet: 14 hosts per subnet.
2. Choose a subnet mask that accommodates the required number of hosts: We need a subnet mask that allows for at least 14 hosts per subnet. The subnet mask 255.255.255.240 allows for 16 IP addresses per subnet, of which 14 can be used for hosts.
3. Determine the number of subnets required: We need 4 subnets.
4. Allocate IP addresses to each subnet:
Subnet 1: 192.168.10.0/28
Subnet 2: 192.168.10.16/28
Subnet 3: 192.168.10.32/28
Subnet 4: 192.168.10.48/28

In this example, we used a subnet mask of 255.255.255.240 to create 4 subnets of 14 hosts each.

### Example 2: Subnetting a Class B Address using FLSM

Suppose we have the Class B IP address 172.16.0.0 and we want to divide it into 8 subnets, each with 30 hosts.

1. Determine the number of hosts required for each subnet: 30 hosts per subnet.
2. Choose a subnet mask that accommodates the required number of hosts: We need a subnet mask that allows for at least 30 hosts per subnet. The subnet mask 255.255.255.224 allows for 32 IP addresses per subnet, of which 30 can be used for hosts.
3. Determine the number of subnets required: We need 8 subnets.
4. Allocate IP addresses to each subnet:
Subnet 1: 172.16.0.0/27
Subnet 2: 172.16.0.32/27
Subnet 3: 172.16.0.64/27
Subnet 4: 172.16.0.96/27
Subnet 5: 172.16.0.128/27
Subnet 6: 172.16.0.160/27
Subnet 7: 172.16.0.192/27
Subnet 8: 172.16.0.224/27

In this example, we used a subnet mask of 255.255.255.224 to create 8 subnets of 30 hosts each.

### Example 3: Subnetting a Class A Address using FLSM

Suppose we have the Class A IP address 10.0.0.0 and we want to divide it into 2 subnets, each with 1000 hosts.

1. Determine the number of hosts required for each subnet: 1000 hosts per subnet.
2. Choose a subnet mask that accommodates the required number of hosts: We need a subnet mask that allows for at least 1000 hosts per subnet. The subnet mask 255.255.252.0 allows for 1024 IP addresses per subnet, of which 1000 can be used for hosts.
3. Determine the number of subnets required: We need 2 subnets.
4. Allocate IP addresses to each subnet:
Subnet 1: 10.0.0.0/22
Subnet 2: 10.0.4.0/22

In this example, we used a subnet mask of 255.255.252.0 to create 2 subnets of 1000 hosts each.

FLSM is a powerful tool for subnetting IP addresses, as it allows network administrators to create multiple subnets with a fixed number of hosts per subnet. This makes it easier to manage IP addresses and network traffic, as well as improve network security.

However, FLSM also has some disadvantages. One of the biggest drawbacks of FLSM is that it can lead to inefficient use of IP addresses. This is because FLSM requires that each subnet have the same number of hosts, which can result in wasted IP addresses in subnets that don’t require as many hosts.

Additionally, FLSM can be inflexible, as it requires that each subnet have a fixed number of hosts. This can make it difficult to adjust the size of subnets as network requirements change over time.

Despite its limitations, FLSM remains a popular subnetting technique in computer networks, especially for smaller networks where IP address management is less complex.

In the next section, we will explore an alternative subnetting technique called Variable Length Subnet Masking (VLSM), which addresses some of the limitations of FLSM.

## VI. Conclusion

In conclusion, Fixed Length Subnet Masking (FLSM) is a subnetting technique that allows network administrators to divide a larger IP address range into multiple smaller subnets, each with a fixed number of hosts. FLSM is a popular subnetting technique in computer networks, especially for smaller networks where IP address management is less complex.

In this article, we discussed the basics of IP addressing and binary representation, the benefits of subnetting, how subnetting works, and the various types of subnetting. We then delved into Fixed Length Subnet Masking (FLSM), discussing its definition, advantages, disadvantages, and how to calculate subnets using FLSM.

We also provided examples of FLSM subnetting for Class C, Class B, and Class A addresses. Finally, we highlighted the importance of subnetting and FLSM in networking, as well as future developments in subnetting and FLSM.

In today’s fast-paced world, where businesses rely heavily on computer networks for their day-to-day operations, proper network management is essential. Subnetting and FLSM are important tools for network administrators to ensure that their networks are efficient, secure, and scalable.

As technology continues to evolve, we can expect further developments in subnetting and FLSM. For example, IPv6, the latest version of the Internet Protocol, introduces a new subnetting technique called Hierarchical IPv6 addressing, which aims to simplify subnetting and improve scalability.

In summary, subnetting and FLSM are essential components of network management, and network administrators must understand and master these techniques to build and maintain efficient, secure, and scalable computer networks.

Thank you for reading this article on Fixed Length Subnet Masking (FLSM) in computer networks. We hope you found it informative and entertaining. If you have any feedback or suggestions, we would love to hear from you. Please feel free to leave a comment below.

WRITTEN BY

## xalgord

Constantly learning & adapting to new technologies. Passionate about solving complex problems with code. #programming #softwareengineering