CIDR Notation Explained: From Basics to Advanced

Classless Inter-Domain Routing (CIDR), pronounced "cider," is the modern standard for representing IP addresses and their associated routing prefixes. It replaced the old classful system (Class A, B, C), which was inflexible and led to rapid exhaustion of IPv4 addresses. CIDR notation is a compact way to specify an IP address and its subnet mask.

How to Read CIDR Notation

A CIDR address looks like this: 192.168.1.0/24.

This notation has two parts:

• The IP Address: 192.168.1.0 is the network address.
• The Prefix Length: /24 is the prefix, which specifies how many of the 32 bits of the IP address are part of the network identifier.

A /24 prefix means that the first 24 bits are the network portion, and the remaining 8 bits (32 - 24) are for host addresses within that network. This corresponds to a subnet mask of 255.255.255.0.

You can use our CIDR Calculator to instantly analyze any CIDR address and see its corresponding network range, subnet mask, and more.

Common CIDR Prefixes

Why CIDR is a Game Changer

The primary benefit of CIDR is its efficiency. It allows for the creation of supernets, where multiple smaller network blocks can be aggregated into a single larger route. This process, known as route summarization, dramatically reduces the size of routing tables on internet routers, which is crucial for the scalability and performance of the global internet.

It also enabled a much finer-grained allocation of IP addresses, slowing the exhaustion of the IPv4 address space. Without CIDR, the internet as we know it would have run out of addresses long ago. For more on optimizing IP allocation, see our article on the power of VLSM.