Introduction To Subnetting
Last Updated : 07 Feb, 2025
Subnetting is the process of dividing a large network into smaller networks called "subnets." Subnets provide each group of devices with their own space to communicate, which ultimately helps the network to work easily. This also boosts security and makes it easier to manage the network, as each subnet can be monitored and controlled separately. In this article, we will discuss Subnetting in detail.
Introduction to Subnet
A subnet is like a smaller group within a large network. It is a way to split a large network into smaller networks so that devices present in one network can transmit data more easily. For example, in a company, different departments can each have their own subnet, keeping their data traffic separate from others. Subnet makes the network faster and easier to manage and also improves the security of the network.
Why Subnetting is Important?
Let's consider a company that follows classful addressing, it has a Class C network (192.168.1.0/24) with 256 IP addresses. It has three departments:
- Sales: 20 devices
- HR: 10 devices
- IT: 50 devices
Without subnetting, all departments share the same network, and all 256 IP addresses are available to everyone, which leads to:
- IP Waste: Only 80 devices are needed (20 + 10 + 50), but all 256 addresses are allocated, wasting 176 addresses.
- Performance Issues: Since all departments are on the same network, any data sent between devices floods the entire network, slowing communication for everyone. For example, heavy data transfer in IT can impact Sales and HR.
- Security Risks: Without subnets, anyone in Sales can access HR or IT devices, exposing sensitive data like payroll systems.
With Subnetting, we split the network into three subnets, allocating just enough IP addresses for each department:
- Sales: 192.168.1.0/27 → 32 IPs (for 20 devices, 12 spare)
- HR: 192.168.1.32/28 → 16 IPs (for 10 devices, 6 spare)
- IT: 192.168.1.48/26 → 64 IPs (for 50 devices, 14 spare)
By subnetting, we:
- Save IP addresses (Efficiency): Only 112 addresses are used (80 + some spare), leaving 144 unused for future growth.
- Keep networks faster (Better Performance): Data within each department stays in its subnet. For example, HR traffic stays in HR, reducing network congestion for Sales and IT.
- Protect sensitive data (Improved Security): Each department is isolated. If someone in Sales tries to access HR systems, subnet restrictions block them.
Key Concepts in Subnetting
IP Addressing
An IP address is made up of different parts, each serving a specific purpose in identifying a device on a network. An IPv4 address consists of four parts called "octets," separated by dots (e.g., 192.168.1.1). It has two main sections:
- Network Portion: Identifies the network the device belongs to.
- Host Portion: Uniquely identifies a device within the network.
IPv4 addresses are divided into classes based on the length of the network and host portions:
- Class A: 8-bit network ID, 24-bit host ID.
- Class B: 16-bit network ID, 16-bit host ID.
- Class C: 24-bit network ID, 8-bit host ID.
For more details, refer to Classful IP Addressing.
What is a Subnet Mask?
A subnet mask is a 32-bit number used in IP addressing to separate the network portion of an IP address from the host portion. It helps computers and devices determine which part of an IP address refers to the network they are present, and which part refers to their specific location or address within that network.
CIDR Notation: A Simplified Approach to Subnetting
Instead of using a long subnet mask (e.g., 255.255.255.0), CIDR uses a simple format like /24. The number after the slash (/n) represents the number of bits used for the network portion of the IP address.
How Subnetting Works?
The working of subnets starts in such a way that firstly it divides the subnets into smaller subnets. For communicating between subnets, routers are used. Each subnet allows its linked devices to communicate with each other. Subnetting for a network should be done in such a way that it does not affect the network bits.
In class C the first 3 octets are network bits so it remains as it is.
- For Subnet-1: The first bit which is chosen from the host id part is zero and the range will be from (193.1.2.00000000 till you get all 1's in the host ID part i.e, 193.1.2.01111111) except for the first bit which is chosen zero for subnet id part.
Thus, the range of subnet 1 is: 193.1.2.0 to 193.1.2.127
Subnet id of Subnet-1 is : 193.1.2.0
The direct Broadcast id of Subnet-1 is: 193.1.2.127
The total number of hosts possible is: 126 (Out of 128, 2 id's are used for Subnet id & Direct Broadcast id)
The subnet mask of Subnet- 1 is: 255.255.255.128
- For Subnet-2: The first bit chosen from the host id part is one and the range will be from (193.1.2.100000000 till you get all 1's in the host ID part i.e, 193.1.2.11111111).
Thus, the range of subnet-2 is: 193.1.2.128 to 193.1.2.255
Subnet id of Subnet-2 is : 193.1.2.128
The direct Broadcast id of Subnet-2 is: 193.1.2.255
The total number of hosts possible is: 126 (Out of 128, 2 id's are used for Subnet id & Direct Broadcast id)
The subnet mask of Subnet- 2 is: 255.255.255.128
The best way to find out the subnet mask of a subnet is to set the fixed bit of host-id to 1 and the rest to 0.
Finally, after using the subnetting the total number of usable hosts is reduced from 254 to 252.
Note:
- To divide a network into four (2 2 ) parts you need to choose two bits from the host id part for each subnet i.e, (00, 01, 10, 11).
- To divide a network into eight (2 3 ) parts you need to choose three bits from the host id part for each subnet i.e, (000, 001, 010, 011, 100, 101, 110, 111) and so on.
- We can say that if the total number of subnets in a network increases the total number of usable hosts decreases.
The network can be divided into two parts: To divide a network into two parts, you need to choose one bit for each Subnet from the host ID part.
In the above diagram, there are two Subnets.
Note: It is a class C IP so, there are 24 bits in the network id part and 8 bits in the host id part.
Example 1: An organization is assigned a class C network address of 201.35.2.0. It uses a netmask of 255.255.255.192 to divide this into sub-networks. Which of the following is/are valid host IP addresses?
- 201.35.2.129
- 201.35.2.191
- 201.35.2.255
- Both (A) and (C)
Solution:
Converting the last octet of the netmask into the binary form: 255.255.255.11000000
Converting the last octet of option 1 into the binary form: 201.35.2.10000001
Converting the last octet of option 2 into the binary form: 201.35.2.10111111
Converting the last octet of option 3 into the binary form: 201.35.2.11111111
From the above, we see that Options 2 and 3 are not valid host IP addresses (as they are broadcast addresses of a subnetwork), and OPTION 1 is not a broadcast address and it can be assigned to a host IP.
Example 2: An organization has a class C network address of 201.32.64.0. It uses a subnet mask of 255.255.255.248. Which of the following is NOT a valid broadcast address for any subnetworks?
- 201.32.64.135
- 201.32.64.240
- 201.32.64.207
- 201.32.64.231
Solution:
Converting the last octet of the netmask into the binary form: 255.255.255.11111000
Converting the last octet of option 1 into the binary form: 201.32.64.10000111
Converting the last octet of option 2 into the binary form: 201.32.64.11110000
Converting the last octet of option 3 into the binary form: 201.32.64.11001111
Converting the last octet of option 4 into the binary form: 201.32.64.11100111
From the above, we can see that in OPTION 1, 3, and 4, all the host bits are 1 and give the valid broadcast address of subnetworks.
and OPTION 2, the last three bits of the Host address are not 1 therefore it's not a valid broadcast address.
Advantages of Subnetting
- It provides security to one network from another network. For example: In an Organization, the code of the Developer department must not be accessed by another department.
- It may be possible that a particular subnet might need higher network priority than others. For example, a Sales department needs to host webcasts or video conferences.
- In the case of Small networks, maintenance is easy.
Disadvantages of Subnetting
- In the case of a single network, only three steps are required to reach a Process i.e Source Host to Destination Network, Destination Network to Destination Host, and then Destination Host to Process.
- In the case of a Single Network only two IP addresses are wasted to represent Network Id and Broadcast address but in the case of Subnetting two IP addresses are wasted for each Subnet.
- The cost of the overall Network also increases. Subnetting requires internal routers, Switches, Hubs, Bridges, etc. which are very costly.
To learn all the basic to advanced subnetting concepts, refer to Subnet Mask Cheat Sheet.
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