An Interactive Guide to IPv4 Subnetting & Calculation

IPv4 subnetting is a fundamental concept in computer networking that allows administrators to divide a large network into smaller, more efficient sub-networks. While the math can seem intimidating, understanding it is key to designing and troubleshooting networks. For a higher-level overview of *why* we subnet, check out our previous post, Demystifying Network Subnetting.

In this guide, we'll dive into the "how" and provide an interactive tool to help you practice and verify your calculations.

The Building Blocks: IP, Mask, and CIDR

Before we calculate, let's quickly recap the components:

• IP Address: A 32-bit number, shown as four octets (e.g., 192.168.1.100), that uniquely identifies a device on a network.
• Subnet Mask: A 32-bit number that separates the IP address into its two main parts: the Network ID and the Host ID.
• CIDR Notation: A shorthand for the subnet mask (e.g., /24), representing the number of leading '1' bits in the mask. A /24 mask is 255.255.255.0.

How to Calculate a Subnet Manually

Let's use an example: 172.16.10.55 /27. Our calculator can do this instantly, but here’s the math behind it.

1. Find the Subnet Mask: A /27 prefix means 27 of the 32 bits are '1's.
   11111111.11111111.11111111.11100000
   In decimal, this is 255.255.255.224.
2. Find the Network Address: This is done by performing a bitwise AND operation between the IP address and the subnet mask.
   172.16.10.55 = 10101100.00010000.00001010.00110111
255.255.255.224 = 11111111.11111111.11111111.11100000
   AND Result = 10101100.00010000.00001010.00100000
   This converts back to 172.16.10.32. This is our Network Address.
3. Find the Broadcast Address: This is found by inverting the subnet mask bits and performing a bitwise OR operation with the network address.
   Inverted Mask = 00000000.00000000.00000000.00011111
   Network Address = 10101100.00010000.00001010.00100000
   OR Result = 10101100.00010000.00001010.00111111
   This converts back to 172.16.10.63. This is our Broadcast Address.
4. Determine Hosts: The number of host bits is 32 - 27 = 5.
   Total Hosts = 2⁵ = 32.
   Usable Hosts = 32 - 2 = 30 (we subtract the network and broadcast addresses).
5. Determine Usable Host Range: The first usable IP is one higher than the network address, and the last is one lower than the broadcast address.
   Range: 172.16.10.33 to 172.16.10.62.

For more technical details on the standards, the original RFC 950 document outlines the procedures for IP subnetting.