Understanding CIDR and Subnetting: A Practical Guide

You see them everywhere in cloud configurations, routers, and firewalls: IP addresses with a slash and a number at the end, like 192.168.1.0/24 or 10.0.0.0/16. This is CIDR notation. If you don't know how to read it, setting up any secure network is guesswork. Subnetting is how we partition a network, and it is a fundamental skill for any network engineer.

The Problem CIDR Solved

In the early days of the internet, IP addresses were divided into Class A, B, and C networks. If you needed 300 IP addresses, a Class C network (256 addresses) was too small, so you had to buy a Class B network (65,536 addresses). This wasted millions of IP addresses. In 1993, RFC 1519 introduced CIDR (Classless Inter-Domain Routing). CIDR allowed custom-sized networks by using variable-length subnet masking (VLSM).

What the Slash Number Means

An IPv4 address consists of 32 bits, divided into 4 octets (each having 8 bits). The number after the slash (e.g., /24) represents the **Prefix Length** — the number of bits reserved for the network portion of the address. The remaining bits (32 minus prefix length) are used for host devices on that network.

For example, in **192.168.1.0/24**:
- 24 bits are for the network.
- 8 bits are left for hosts (32 - 24 = 8).
- 8 bits allows for 2⁸ = 256 total IP addresses.

The Reserved Addresses

In every subnet, you cannot use all the available IP addresses for devices. Two addresses are always reserved:
1. **Network Address:** The first address in the block (e.g., 192.168.1.0). It represents the subnet itself.
2. **Broadcast Address:** The last address in the block (e.g., 192.168.1.255). It is used to send data to all hosts on the subnet.
Therefore, the number of usable host addresses is always **2^{(32 - prefix)} - 2**.

Common CIDR Prefix Cheat Sheet

Here is a quick reference for common prefix sizes:
- **/30:** 4 total IPs (2 usable). Often used for point-to-point router links.
- **/28:** 16 total IPs (14 usable). Useful for small office departments.
- **/24:** 256 total IPs (254 usable). The standard size for most local home networks.
- **/16:** 65,536 total IPs (65,534 usable). Typically used for cloud VPC environments.