IPv4 CIDR Notation and Subnet Masks

Title: IPv4 CIDR Notation and Subnet Masks: Mastering Addressing and Subnetting


IPv4 (Internet Protocol version 4) addresses are fundamental to the functioning of the internet, enabling devices to communicate across networks. CIDR (Classless Inter-Domain Routing) notation and subnet masks play a crucial role in efficiently allocating and managing IPv4 address space. In this technical guide, we will explore the concepts of CIDR notation and subnet masks, providing a comprehensive understanding of their significance in IP addressing and subnetting.

1. Understanding CIDR Notation

CIDR notation represents an efficient and concise way to express IP address ranges. It uses the format “IP address/network prefix,” where the network prefix indicates the number of bits used for network identification. For example:

  • represents the IP address with a subnet mask of, where 24 bits are reserved for the network portion.
  • represents the IP address with a subnet mask of, where 8 bits are allocated for the network.

2. Subnet Masks and Addressing

A subnet mask defines the network portion and host portion of an IP address. It consists of a series of 1s followed by a series of 0s. The number of 1s indicates the size of the network prefix, while the number of 0s represents the number of available host addresses within the subnet.

3. Subnetting and Network Segmentation

Subnetting is the process of dividing an IP network into smaller, more manageable subnetworks. It allows network administrators to efficiently allocate IP addresses, optimize routing, and enhance security. Using CIDR notation, network administrators can easily identify the network prefix and calculate the number of available host addresses in each subnet.

4. Calculating Subnets and Hosts

To determine the number of subnets and hosts available in a given CIDR notation, follow these steps:

  • Identify the subnet mask in CIDR notation (e.g., /24, /28).
  • Subtract the CIDR notation from 32 (e.g., for /24, 32 – 24 = 8).
  • Calculate the number of subnets as 2 to the power of the remaining bits (e.g., for /24, 2^8 = 256 subnets).
  • Calculate the number of hosts per subnet as 2 to the power of the host bits (e.g., for /24, 2^8 – 2 = 254 hosts per subnet, subtracting 2 for the network and broadcast addresses).

5. CIDR and Routing Efficiency

CIDR notation enables efficient utilization of IPv4 address space and improves routing efficiency. By aggregating multiple IP address blocks into a single CIDR notation, internet routing tables become more compact, reducing the size and complexity of routing tables across the internet.


CIDR notation and subnet masks are essential tools for managing IPv4 address space, facilitating network segmentation, and optimizing routing efficiency. Understanding CIDR notation empowers network administrators to design robust, scalable, and well-organized networks while conserving valuable IPv4 resources.

By mastering CIDR notation and subnetting, IT professionals can effectively manage IP address allocations, ensuring that networks run smoothly and efficiently in an ever-evolving digital landscape.