Understanding subnetting in IPv4 networks

Subnetting is a fundamental concept in IPv4 networking that allows administrators to efficiently manage and allocate IP address spaces. In this blog post, we will explore subnetting in IPv4 networks, including the underlying concepts, techniques, benefits, and how to perform subnet calculations.

Table of Contents

Subnetting Basics

Subnetting is the process of dividing an IP address space into smaller subnetworks, called subnets. Each subnet represents a logical grouping of devices that can communicate with each other directly, without the need for a router. Subnetting helps network administrators manage IP address allocation more efficiently, control network traffic, and improve security by isolating different segments of the network.

An IPv4 address consists of 32 bits, which can be divided into two parts: the network prefix and the host identifier. The network prefix identifies the specific network, while the host identifier identifies the device within that network. The boundary between the network prefix and the host identifier is determined by the subnet mask.

Subnet Mask

A subnet mask is a 32-bit number that shares the same format as an IPv4 address. It is used to distinguish the network prefix and host identifier in an IPv4 address. The subnet mask contains a continuous sequence of 1's, followed by a continuous sequence of 0's. The 1's in the subnet mask correspond to the bits in the network prefix, while the 0's correspond to the bits in the host identifier.

For example, a subnet mask of (binary: 11111111.11111111.11111111.00000000) indicates that the first 24 bits of the IPv4 address represent the network prefix, while the remaining 8 bits represent the host identifier.

Subnet Calculations

Performing subnet calculations is an essential skill for network administrators, as it helps determine the appropriate subnet mask, the number of available subnets, and the number of hosts per subnet. Here's a step-by-step guide to performing subnet calculations:

  1. Determine the number of bits needed for the host identifier: Calculate the number of host bits required to accommodate the desired number of hosts per subnet, including the network and broadcast addresses. To do this, use the formula 2n ? desired hosts + 2, where n is the number of host bits.
  2. Calculate the subnet mask: Subtract the number of host bits from 32 to determine the number of network bits. Then, create a subnet mask by filling the network bits with 1's, followed by the host bits with 0's. Convert the binary subnet mask to decimal format.
  3. Calculate the number of subnets: Determine the number of bits borrowed from the host identifier to create the subnets. Use the formula 2m, where m is the number of borrowed bits, to calculate the number of available subnets.
  4. Calculate the number of hosts per subnet: Use the formula 2n - 2, where n is the number of host bits, to calculate the number of usable hosts per subnet. The subtraction of 2 accounts for the network and broadcast addresses, which cannot be assigned to individual hosts.
  5. Identify the subnet ranges: Calculate the subnet ranges by incrementing the network prefix for each subnet. The first address in each range is the network address, and the last address is the broadcast address. The remaining addresses are available for hosts.

Classless Inter-Domain Routing (CIDR)

Classless Inter-Domain Routing (CIDR) is an IP addressing scheme that improves the efficiency of IP address allocation and simplifies routing. CIDR eliminates the concept of address classes (A, B, and C) and allows for variable-length subnet masks, enabling more flexible and efficient subnetting.

In CIDR notation, an IPv4 address is followed by a slash (/) and the number of network bits in the subnet mask. For example, the address with a subnet mask of can be represented in CIDR notation as, indicating that the first 24 bits represent the network prefix.

Benefits of Subnetting

Subnetting offers several benefits for IPv4 networks, including:

5 min. read
15 Oct 2022

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