Understanding Subnet Masks

Understanding Subnet Masks

Jul 12, 2023·

6 min read


A subnet mask is a crucial component of IP addressing in computer networks, particularly in IPv4. It divides an IP address into network and host portions, allowing efficient routing of data packets. This article explores the significance of subnet masks, their relationship with IP addressing, and their importance in network communication. It also delves into the structure, format, and binary representation of subnet masks, highlighting their conversion between decimal and binary notations.

What is an IP Address

An IP address, short for Internet Protocol address, is a unique numerical label assigned to each device connected to a computer network that uses the Internet Protocol for communication. It serves as an identifier for devices, allowing them to send and receive data across the internet or within a local network. An IP address consists of a series of four sets of numbers separated by periods (e.g., The two most commonly used IP versions are IPv4, which uses 32-bit addresses, and IPv6, which uses 128-bit addresses. IP addresses enable devices to establish connections, communicate, and route data packets to their intended destinations on the internet. They play a fundamental role in network communication and are essential for various internet-based services and applications

Components of an IP Address


An IP address is divided into four sets of numbers called octets. Each octet represents eight bits and ranges from 0 to 255. The octets are separated by periods (dots) to create the familiar decimal notation. For example, in the IP address, the octets are 192, 168, 0, and 1.

Network Part

The network part of an IP address is the portion that identifies the network to which the device belongs. It represents the network address and remains the same for all devices within the same network. The length of the network part depends on the class of the IP address or the subnet mask associated with it. The network part identifies the network segment that the device is connected to and enables routing between networks.

Host Part

The host part of an IP address distinguishes individual devices within a network. It represents the unique identifier of a device within its network segment. The length of the host part depends on the network requirements and the number of hosts within the network. Each device within a network must have a different host part to ensure uniqueness.

Subnet Masks

Subnet masks are vital for dividing IP addresses into network and host portions. They enable network devices to identify the network to which an IP address belongs. Subnet masks play a crucial role in efficient network communication and routing. They allow administrators to divide large IP address spaces into smaller subnets, implementing rules and policies for secure communication and efficient routing of data packets. Subnet masks also contribute to better address utilization by hierarchically allocating addresses within subnets.

Determining Network and Host Portions

The subnet mask consists of a series of 1s followed by a series of 0s. The number of 1s in the subnet mask determines the size of the network portion, while the number of 0s determines the size of the host portion.

For example, an IP address of with a subnet mask of has 24 bits set to 1, indicating that the first 24 bits represent the network portion, and the remaining 8 bits represent the host portion.

Classful vs Classless Subnet Masks

Classful addressing and default subnet masks were used in early versions of IP addressing to allocate addresses based on fixed address classes (A, B, C, D, and E). However, CIDR was introduced to overcome the limitations of classful addressing. CIDR introduced classless subnet masks, allowing for more flexible address allocation and routing efficiency. Subnet masks can be represented in decimal or CIDR notation, with CIDR notation being more commonly used in modern networking.

Subnet Mask Calculation Using Classes

In traditional IP addressing, IP addresses were divided into different classes (A, B, C, D, and E), each with its default subnet mask. For example, in Class C addresses like, the default subnet mask is This means the first three octets (24 bits) represent the network portion, allowing for up to 254 hosts on a single network. However, subnetting within a Class C network provides the ability to create smaller subnets by borrowing bits from the host portion.

For instance, by borrowing 2 bits, the subnet mask changes to In this case, the first three octets (24 bits) still indicate the network portion, while the last octet has 2 bits for subnets and 6 bits for hosts. This allows the creation of 4 subnets, each capable of accommodating up to 62 hosts. By borrowing additional bits, even smaller subnets with fewer hosts can be configured, providing greater flexibility in network design and more efficient utilization of IP addresses.

Subnet Mask Calculation Using CIDR

CIDR (Classless Inter-Domain Routing) notation is a method of IP addressing that allows for more flexible allocation of IP addresses and efficient routing. It combines an IP address with a prefix length, indicating the number of bits used to represent the network portion. For example, an IP address with a CIDR notation of means that the first 24 bits represent the network portion.

To calculate the subnet mask using CIDR, the prefix length is converted into a binary string. For a prefix length of 24, the binary representation would be "11111111.11111111.11111111.00000000". This binary string is then converted back to decimal form, resulting in a subnet mask of "".


Subnetting is a valuable technique used in IP networking to divide a larger network into smaller subnetworks, known as subnets. It offers benefits such as efficient IP address utilization, improved network performance, and enhanced security. By borrowing bits from the host portion of an IP address, subnetting allows for the creation of multiple smaller subnets within the original network.

For example, let's consider a network with the IP address and a default subnet mask of By subnetting, we can divide this network into smaller subnets. Suppose we borrow two bits from the host portion, increasing the network prefix from 24 bits to 26 bits. The resulting subnet mask becomes, where the last two bits represent the subnets, and the remaining six bits represent the hosts within each subnet.

Through subnetting, the original network can be efficiently divided into multiple subnets, each with its IP address range. This allows for better IP address management, improved network organization, and enhanced performance.

Benefits of Subnet Masks and Subnetting

  1. Efficient IP address allocation: Subnet masks ensure the efficient utilization of IP addresses by dividing a network into smaller subnets, preventing wastage and optimizing address space.

  2. Improved network performance: Subnetting reduces network congestion and enhances traffic flow by dividing a large network into smaller subnets, localizing network communications and improving overall performance.

  3. Enhanced network security: Subnetting provides network segmentation, isolating different segments to limit the impact of security breaches, control network access, and ensure better data flow control.

  4. Simplified network management: Subnetting simplifies network management by dividing a large network into manageable segments, allowing for customized configurations, policies, and efficient troubleshooting.


Subnet masks are fundamental components of IP addressing and play a crucial role in network communication, routing, and address allocation. They allow for the division of IP addresses into network and host portions, enabling efficient routing and secure communication. Subnet masks facilitate subnetting, summarization, and aggregation techniques, contributing to scalable network architectures. Understanding subnet masks, their structure, conversion between decimal and binary notations, and their impact on network communication is essential for network administrators. By following best practices and effectively configuring subnet masks, organizations can optimize network performance, enhance security, and ensure efficient address utilization.