Designing a Basic Subnetting Scheme

Welcome to the next lesson in our TCP/IP module. In our previous session, you mastered the essential skill of analyzing a given IP address and subnet mask to determine its network address, broadcast address, and the range of usable IPs. You learned the "magic number" method to do this quickly and accurately.

Today, we shift from analysis to design. Instead of examining a single subnet, you will learn how to take a large block of IP addresses and divide it into multiple smaller networks. This is a fundamental task for any network engineer, allowing you to create organized, secure, and efficient network structures. This lesson directly addresses your goal of mastering "easy subnetting" by applying the calculation skills you've already built to a practical design scenario.

Why Divide a Network?

Before we dive into the "how," let's quickly cover the "why." Subnetting isn't just a mathematical exercise; it's a critical tool for network management. In your work with network security and engineering, you'll divide networks to achieve several key goals.

The Core Concept: Borrowing Bits for Networks

In the last lesson, we saw that a subnet mask splits an IP address into a network portion and a host portion. To create more networks, we must extend the network portion by "borrowing" bits from the host portion.

The key question is: How many bits do we need to borrow?

The answer depends on the number of subnets you need. The relationship is governed by a simple formula, where n is the number of bits you borrow:

If you need to create 5 subnets, you must find the smallest power of 2 that is greater than or equal to 5.

• $2^1=2$ (Not enough)
• $2^2=4$ (Still not enough)
• $2^3=8$ (This works! We need to borrow 3 bits.)

This will create 8 subnets, and you can use 5 of them for your design, leaving 3 for future expansion.

This video provides an excellent visual demonstration of "borrowing" or "hacking" bits from the host portion to create the networks you need.

The Subnetting Design Process

With this core concept in mind, designing a basic subnetting scheme follows a clear, four-step process.

1. Determine the bits to borrow: Based on your required number of subnets, use the $2^n$ formula to find the number of bits (n) you must borrow from the host portion.
2. Calculate the new subnet mask: Add the number of borrowed bits (n) to the original CIDR prefix. For example, if you start with a /24 network and borrow 3 bits, your new subnets will all use a /27 prefix. Convert this to dotted-decimal notation (a /27 is 255.255.255.224).
3. Find the block size (increment): Just like in our last lesson, use the new subnet mask to find the block size. Block Size = 256 - (value of the interesting octet). This tells you the increment for your new network addresses.
4. List the subnet ranges: Starting from the original network address, list the new subnets by adding the block size to the "interesting octet" for each subsequent network.

The image below provides a fantastic visual summary of this entire process, showing a /24 network being divided into 8 smaller /27 subnets by borrowing 3 bits.

A Fully Worked Example

Let's solidify this with a step-by-step written example. This is the exact process you'll use in real-world scenarios and on certification exams.

Practice Makes Permanent

Now it's your turn to be the network designer. Use the four-step process to solve the following problem.

Problem: You are the network administrator for a small company and have been assigned the network address block 172.25.0.0 /16. You need to create at least 12 separate subnets for different departments (Sales, Marketing, Engineering, etc.).

Your Task:

1. How many bits do you need to borrow?
2. What will the new subnet mask be in CIDR and dotted-decimal notation?
3. What is the block size (increment), and in which octet does it apply?
4. List the first five new subnets, including their Network Address, full Usable Host Range, and Broadcast Address.

Conclusion

In this lesson, you've moved from simply analyzing subnets to designing them. You learned how to take a network requirement—the need for a certain number of subnets—and translate it into a concrete IP addressing scheme. By borrowing bits from the host portion, you can create new, smaller networks, each with its own distinct range of addresses. This skill is foundational for building logical, secure, and high-performing network infrastructures.

Key Takeaways:

• Subnetting is done for security, performance, and organization.
• The number of subnets you can create is determined by the formula $2^n$, where n is the number of bits you borrow from the host portion.
• Borrowing n bits adds n to the original CIDR prefix, creating a new, longer subnet mask for all the new subnets.
• The "block size" method from the previous lesson is used to find the network addresses of the new subnets based on the new mask.

In our next lesson, we will take this concept a step further with Variable Length Subnet Masking (VLSM). You'll learn how to subnet a subnet, allowing you to create networks of different sizes to match specific needs and use your IP address space even more efficiently.