The ability to subnet a network (i.e. divide a network into smaller units) is a core technical skill. It is important for network engineers/administrators. Subnetting is also one of the skills that is often easily misunderstood. Many network problems or security failures can be traced to poorly executed Subnetting skills. Subnetting Practice is important.

This post will help you understand the importance of Subnetting, learn some basics of Subnetting, and help you explore some of the best subnetting practice resources.

You will also find the materials listed in the article to be very helpful if you are preparing for Certification Exams like the Cisco Certified Network Associate Exam (CCNA 200-301) or other similar Networking exams. I used these materials to help me prepare for my CCNA Exams – where you have to solve subnetting questions in less than a minute without using a calculator.

What Does Subnetting Mean?

Subnetting means dividing a large network into smaller units called “subnets”. This division takes place at layer 3 (Network layer) of the OSI Model and involves splitting a large block of IP addresses into smaller units. IP addresses are important when you want to send messages (called packets) from one network to another.

It is good practice to subnet a network so that it will be easier to manage and also to achieve some goals such as security and cost reduction (Getting a block of IP costs money, so you would want to use them efficiently).

Imagine a mail delivery man who is out to deliver different parcels (packets) to different recipients in a large City, His job will be a lot easier if the City is divided into different “Zones” identified by different codes, well-organized streets, house numbers, etc This is way better than having the City with no delineations or clear markings. The Mail Delivery man can deliver packages to different recipients’ addresses efficiently because of the organization of the city. The same is true of subnetting. It helps organize your network so that the delivery of packets (messages) will be more efficient, and secure.

Why Is Subnetting Important

There are several reasons why subnetting is important. These include the following:

1. Efficient Management of IP Addresses

When the Internet was created, no one envisaged that there would be an explosion of devices connected to it. Therefore, only a limited number of IPv4 addresses were created – about 4 billion IPv4 addresses. This is far below what is needed today. Subnetting makes it possible to use only a small block of IP addresses to power several devices.

It prevents the wastage of IP addresses. Normally, a company will request a block of IP addresses from IANA based on the size of the company, With subnetting, a smaller block can be assigned to them with minimal wastage and they can efficiently manage such by subnetting to meet their goals. This helps save costs and efficiently manage the available IP addresses.

2. Optimization of Network Performance

Subnetting helps in the Optimization of Network performance. A large network is prone to breakages or clogging as packets (messages) may be sent across the entire network instead of only a small segment. Subnetting ensures that messages do not have to traverse the entire network when they are only needed in a given segment.

Routing is also more efficiently done with subnetting, the shortest and better routes to package delivery are created with subnetting, thereby improving performance. It is also easier to scale a network when it is subnetted, remove a subnet when it is no longer needed, and create new subnets when more devices are connected, all without breaking the entire network.

3. Security

Subnetting can be used to achieve security goals. You subnet a network so that certain sensitive messages are only limited within specific subnets and not accessible or sent to other subnets. Subnetting is therefore a tool for Network administrators and security experts to implement Access control policies and firewall rules for enhanced network security.

This also ensures that security breaches -when they occur -are limited to certain segments or subnets of the network.

4. Troubleshooting is Better with Subnetted Networks

Troubleshooting is the process of diagnosing the cause of a failure or poor performance. It is much more difficult when you are dealing with a large network. But with subnetting, it is easier to narrow the troubleshooting to the smaller network that is operating poorly and therefore easier to fix. Also, subnetting will normally ensure that problems in a subnet are limited to that subnet and therefore easier to fix.

Types of Subnetting

Subnetting practice. IP Addresses are in "Blocks"

There are two broad types of subnetting, or more appropriately, two broad techniques for achieving subnetting. These are Classful Subnetting (also known as Fixed Length Subnetting) and Classless Subnetting (Variable Length Subnet Maskin) or simply VLSM.

1. Classful Subnetting (Fixed Length Subnetting)

This is the more traditional technique for subnetting a network. It is not a very effective method of subnetting as it still leads to lots of wastage of IP addresses. For this reason, it is often not used in modern networks.

It relies on the Classes of IP addresses – Class A, Class B, and Class C – hence the name “classful”. (There are 5 classes of IP addresses, Class A, B, C, D, and E., Class D and E are not included in subnetting as they are reserved for multicast and experimentation purposes respectively).

Each of the Classes of IP addresses has a default subnet mask and new subnets are created by “borrowing” bits from the host portion (i.e. from the right) of an IP address to create new subnets. The result is that the sizes of all subnets are the same and have an equal number of hosts for each subnet. There is limited flexibility and the number of subnets is limited to just Class A, B, and C.

Class A Subnetting

Subnetting for Class A has a default mask of (or /8 notation) and the IP address range is to The first 8 bits are the network portion (the /8) while the other 3 octets or 24 bits represent the host portion. Using this default subnet mask,

The total number of subnets for Class A is calculated as 2^(number of network bits) or 256 subnets. while the total number of usable hosts is calculated as 2^(number of host bits) -2, that is 2^24 – 2 or 16,777,214 hosts.

The first address is usually not used as it is the network address and the last is also not assigned to a host because it is used as the broadcast address.

Class B Subnetting

Subnetting for Class B has a default mask of (or /16 notation) and the IP address range is to The first 16 bits are the network portion (indicated by the /16) while the other 2 octets or 16 bits represent the host portion. Using this default subnet mask,

The total number of subnets for Class A is calculated as 2^(number of network bits), that is 2^16 or 65,536 subnets. while the total number of usable hosts is calculated as 2^(number of host bits) -2, that is 2^16 – 2 or 65,534 hosts.

Class C Subnetting

Subnetting for Class A has a default mask of (or /8 notation) and the IP address range is to The first 24 bits are the network portion (indicated by the /24) while the last octets or 8 bits represent the host portion. Using this default subnet mask,

The total number of subnets for Class A is calculated as 2^(number of network bits), that is 2^24 or 16,777,216 subnets. while the total number of usable hosts is calculated as 2^(number of host bits) -2, that is 2^8 – 2 or 254 hosts.

Class A has fewer subnets but a very large number of hosts per subnet, and Class C has a very large number of subnets but fewer number of hosts per subnet.

2. Classless Subnetting (Variable Length Subnet Masking)

The Classless Subnetting technique or type does not recognize the strict boundaries of the Class A, B, or C range of IP addresses. This method of subnetting was invented to solve the inefficiencies (IP address wastages) inherent in Classful Subnetting.

VLSM subnetting technique is very flexible and allows the assignment of IP addresses based on the needs of the subnet. You can assign a large number of IP addresses as easily as you can assign a small number. It is efficient and suitable even for complex numbers.

Classless Subnetting or VLSM uses a Classless Interdomain Routing or CIDR to denote different subnet masks. CIDR uses classes other than those normal for Class A, B, or C networks. For example, we can have a /20, /27, /9, etc. network masks. You will find the following material very valuable to learn how to subnet a network.

Subnetting Resources/materials


This is a great practice tool for those preparing to sit for the CCNA exams. It has gained some promotion from Neil Anderson. Neil is a “goto” guy for CCNA exam courses. You will also find other materials such as subnetting calculators (You cannot use a calculator in the exam but are very useful on the job), subnetting cheat sheets, CIDR (Classless Inter-Domain Routing) calculations, etc. You can also download the subnetting Cheatsheet

Download the Subnetting Cheat Sheet

The cheat sheet is an excellent resource for those preparing for Networking certification exams such as the CCNA. You cannot use the software or a calculator so cheat sheets are very helpful for the exam.


Boson ExSim is one the best exam practice preparation packages for the CCNA 200-301. They offer a free subnetting guide for download. It is particularly useful for those who are preparing to sit for a networking exam such as the CCNA. The materials are very descriptive and contain tables and guides for you to memorize to answer exam questions quickly. Boson ExSim Subnetting practice for free. You will find this very useful for the CCNA 200-301

Download a free copy of Boson Subnetting Practice below

Click Here To Download the Subnetting Practice pdf



This is specifically for those preparing for the Cisco CCNA 200-301 Exam. It gives a step-by-step process on how to create subnets, VLSM, CIDR, number of hosts, number of subnets, etc., and lots of examples to help you prepare for the exams.

4. is a great site to practice. The questions are simple and much more concise (compared to the earlier option). It will teach you to solve subnetting questions “with your eyes” within seconds. You can solve subnetting many less complicated problems within seconds.


This is very similar to the one above. You will find it rewarding to practice subnetting. And like the others, it auto-generates the questions. You will find endless streams of questions to work with. The questions are somewhat easier to answer but they do finetune your subnetting skill. Not only does it provide the right answer but also further explanation. This you can see by clicking on the “see explanation” at the bottom.

Here is an example of the type of questions you find on the site. It shows the answer to the question below the question.

Question: You are designing a subnet mask for the network. You want 40 subnets with up to 600 hosts on each subnet. What subnet mask should you use?


Next question

Make sure you do enough practice to understand subnetting. Do not relent until you master it. It gets easier with more practice.


Subnetting is an important topic, particularly for network engineers. It is the process of splitting large IP blocks into smaller blocks. Subnetting a network is important for network optimization, efficient management of IP addresses, and network security. Some of the best subnetting practice materials are,,, Study-CCNA.COM, and the Boson Exsim Subnetting Guide. Practicing with some of these subnetting resources will help you master the skill.

Check out some Free CCNA Course material in the link below:


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