What CIDR is
Classless Inter-Domain Routing (CIDR) is the modern way to describe blocks of IP addresses. A CIDR notation pairs a base address with a prefix length after a slash, such as 192.168.1.0/24. The number after the slash says how many leading bits identify the network, and the rest identify hosts within it.
How subnetting works
The prefix length controls the size of the block:
/24reserves 24 bits for the network, leaving 256 addresses, of which 254 are usable hosts./16leaves far more host addresses, around 65,000./32refers to a single address.
Subnetting splits a larger block into smaller ones. For example, a /24 can be divided into two /25 networks, each serving a separate group of devices while keeping traffic organized and secure.
Why it matters
CIDR replaced the old rigid class system and lets networks be sized exactly to need, which conserves scarce IPv4 addresses and keeps routing tables compact. Network engineers use it daily to plan address space, write firewall rules, and define routes. For example, allowing access from 10.0.5.0/24 permits an entire department's subnet in one rule. The WhatIP subnet-calculator tool computes host counts, ranges, and masks from any CIDR block.