Understanding IP Subnetting and CIDR Notation

By Sanjeev Nair · Published May 2026

Networking doesn't have to be confusing. Demystify Classless Inter-Domain Routing (CIDR), network masks, and binary host calculations with this practical guide.

What is an IP Address and Subnetting?

An **IPv4 address** is a 32-bit binary number, typically written as four decimal octets separated by dots (e.g., `192.168.1.1`). Subnetting is the practice of dividing a physical network into smaller, logical sub-networks (subnets). This division improves security, reduces broadcast traffic, and helps manage IP address allocation efficiently.

The Role of the Subnet Mask

Every IP address on a network is accompanied by a **subnet mask**. The mask tells network routing hardware which part of the 32-bit IP address belongs to the **Network Portion** (identifying the specific street) and which part belongs to the **Host Portion** (identifying the individual house).

A subnet mask is also a 32-bit binary number. Standard subnet masks consist of continuous binary `1`s followed by continuous binary `0`s:

• Binary `1`s represent the network address space.
• Binary `0`s represent the space available for host devices (computers, servers, routers).

Dotted Decimal vs. CIDR Notation

Traditionally, subnet masks were written in dotted decimal notation, such as `255.255.255.0`. Today, **CIDR (Classless Inter-Domain Routing)** is the industry standard. CIDR represents the mask by appending a slash followed by the count of active network bits (the binary `1`s).

How Subnet Math Works

When calculating network values, routers use binary logical operations:

• Network Address: Calculated by performing a bitwise logical **AND** operation between the IP address and the subnet mask. The result isolates the network portion.
• Broadcast Address: Calculated by performing a bitwise logical **OR** between the IP address and the bitwise inversion of the subnet mask (known as the wildcard mask).
• Usable Hosts: Calculated using the formula $2^H - 2$, where $H$ is the number of host bits (binary zeros in the mask). We subtract $2$ because the very first address is reserved to identify the network itself, and the final address is reserved for broadcast messages.

Step-by-Step Example Calculation

Let's find the subnet parameters for the IP `192.168.1.100` with mask `/26` (which corresponds to `255.255.255.192`):

1. Determine host bits: A `/26` mask uses 26 bits for the network, leaving $32 - 26 = 6$ host bits.
2. Calculate total addresses: $2^6 = 64$ total addresses.
3. Calculate usable hosts: $2^6 - 2 = 62$ usable host addresses.
4. Find Network boundary: In binary, the last octet of the IP (100) is `01100100`. The mask octet (192) is `11000000`. Performing logical AND:
   01100100 & 11000000 = 01000000 (which is 64 in decimal). So the network address is **192.168.1.64**.
5. Find Broadcast boundary: Setting all 6 host bits to binary `1` gives the binary octet `01111111` (which is 127 in decimal). So the broadcast address is **192.168.1.127**.
6. Identify Usable Range: The address right after the network address and up to the address right before the broadcast address. In this case, **192.168.1.65** to **192.168.1.126**.