Subnetting Tricks: Subnetting Made Easy with Examples

Subnetting is a process of breaking a large IP subnet into multiple smaller IP subnets. There are many traditional methods of subnetting. The only problem with them is that they take a long time to perform subnetting. If you have ample time, you can use any of them to subnet your IP subnet. However, if you want to perform subnetting rapidly and accurately in an exam or interview, you need modern methods of subnetting. This tutorial presents these methods. These methods let you solve subnetting questions in less than a minute.

There are three types of subnetting questions you may face in an exam or interview.

1. Calculate the number of networks and host addresses in a given IP subnet.
2. Find the network ID and broadcast ID of a given IP subnet.
3. Identify the subnet mask of a given IP subnet.

Calculating networks and host addresses

In these types of questions, you need to calculate the total number of IP subnets and host addresses you can get from a given IP subnet. The following table lists some IP subnets. Calculate the total number of subnets you can obtain from each subnet, and the total number of host addresses and valid host addresses in each subnet.

In the first step, determine the IP class of the given IP subnet. An IP address consists of 32 bits. These bits are divided into four sections. Sections are separated by a dot. Each section contains 8 bits. 8 bits are equal to 1 byte or 1 octet.

Use the first octet's value to determine the IP class.

An IP address contains network and host bits. In slash notation, the number of network bits is written with the IP address after the slash.

If you subtract the network bits from the total IP bits, you get the host bits.

Host bits  = 32 - Network bits

Each IP class has reserved network bits in all IP addresses. You cannot use reserved network bits for subnetting. You can use only the host bits for subnetting.

If you subtract the given network bits from the reserved network bits, you get the host bits used in subnetting.

Number of host bits used in subnetting = Total bits used in network portion – Reserved bits for the network portion

Once you know how many host bits are used as network bits to create additional networks, use the following formulas to calculate the number of networks, the total number of hosts, and the number of valid hosts.

Number of networks (subnet) = 2N
Number of total host addresses in each network = 2H
Number of valid host addresses in each network = 2H – 2

Here, N is the number of host bits used as network bits to create additional subnets, and H is the available host bits.

Finding address type, network ID, and broadcast ID

In these types of questions, you need to find three things: the address type, the network ID, and the broadcast ID from the given IP address. Let us take some example IP addresses and find their address type, network ID, and broadcast ID.

In the first step, find the interesting octet. The interesting octet is the octet in which the network address separates from the host address in the IP address.

If you know the number of network bits, you can easily find the octet in which the network bits are separated from the host bits. For example, if an IP address has a /15 subnet mask in slash notation, it has 15 network bits. The number 15 comes in the second octet's range (9 - 16). Hence, the interesting octet will be the second octet.

After identifying the interesting octet, follow these steps to determine the IP address type.

• If the octet comes before the interesting octet, write it as it is.
• If the octet comes after the interesting octet, write 0 in the network address and 255 in the broadcast address.
• In the interesting octet, write a placeholder character X. You will calculate its value in the next step.

Subtract the number of network bits (value after slash) from the ending number of the interesting octet's range.

• Subtract it from 8 if the interesting octet is the first octet.
• Subtract it from 16 if the interesting octet is the second octet.
• Subtract it from 24 if the interesting octet is the third octet.
• Subtract it from 32 if the interesting octet is the fourth octet.

In the next step, calculate the block size. Use the remainder of the previous subtraction to calculate the block size.

Block size = 2 Remainder of the subtraction

In the next step, count in blocks until the interesting octet's value falls within the block's range. Each block size has a range of numbers. The interesting octet's value must be in this range. For example, if the interesting octet's value is 27 and the block size is 8, the range will be 24-31 (0-7, 8-15, 16-23, 24-31).

The block size range starts from 0 and increases in multiples of the block size. For example, if the block size is 4, the numbers in the first range will be 0, 1, 2, and 3.

In the next step, update the placeholder value as follows.

• In the network address, replace the placeholder with the range's starting value.
• In the broadcast address, replace the placeholder value with the ending value of the range.

After updating the placeholder value, you can easily determine the address type of the given address.

• If the given address exactly matches the network address, it is a network address.
• If the given address exactly matches the broadcast address, it is a broadcast address.
• If the given address is neither a network address nor a broadcast address, it is a valid host address.

Calculating the subnet mask of a given IP address

In these types of questions, you calculate the subnet mask of a given IP address. Let us take some example IP addresses and calculate their subnet masks.

20.10.30.0/8, 111.187.45.34/14, 162.160.46.24/20, 202.100.20.50/27

In the first step, find the interesting octet.

In the second step, use a temporary subnet mask based on the interesting octet.

• If the octet comes before the interesting octet, write 255.
• If the octet comes after the interesting octet, write 0.
• In the interesting octet, write a placeholder character X.

Subtract the given network number (value after slash) from the interesting octet's range's ending number.

Use the following formula to calculate the interesting octet's value.

Interesting octet's value = 256 - 2Remainder of the subtraction

Replace the placeholder value with the interesting octet's value. It returns the subnet mask for the given IP address.

Conclusion

Subnetting is a fundamental skill. Almost all entry-level networking exams and interviews include subnetting-related questions. By understanding how to quickly identify network classes, calculate network and host addresses, and determine subnet masks, you can solve any subnetting problem with confidence and accuracy. With continued practice using the techniques presented in this tutorial, you can solve complex subnetting questions swiftly and efficiently.