Understanding CIDR Notation

Introduction to CIDR

CIDR, or Classless Inter-Domain RoutingA method for allocating IP addresses and routing Internet Protocol packets. It was introduced in 1993 to replace the previous classful network addressing architecture., is a method for allocating IP addresses and routing Internet Protocol packets. It was introduced in 1993 to replace the original classful network addressing system (Class A, B, C) which was inefficient and led to rapid IPv4 address exhaustion.

At its core, CIDR allows for more flexible and efficient use of IP addresses by enabling variable-length subnet masking (VLSM) and route aggregation. This means network administrators can create subnets of varying sizes, perfectly matching the needs of their networks, rather than being restricted to fixed class sizes.

The Structure of CIDR

A CIDR block is represented as an IP address followed by a slash and a decimal number, like 192.168.1.0/24. This notation combines two crucial pieces of information:

IP Address: This is the standard 32-bit IPv4 address (e.g., 192.168.1.0). It can represent a network address or a host address within that network.
Prefix Length: The number after the slash (e.g., /24) indicates how many bits of the IP address are used for the network portion. This is also known as the subnet mask.

Visualizing the CIDR Structure

The prefix length determines the boundary between the network and host parts of an IP address.

Why CIDR is Essential

CIDR brought significant improvements to IP addressing and routing:

Address Space Conservation: By allowing flexible subnetting, CIDR prevents the waste of IP addresses that occurred with classful addressing. Instead of assigning a large Class B network (65,534 hosts) to an organization that only needed a few hundred, CIDR allows for a more precise allocation.
Routing Table Aggregation (Supernetting): CIDR enables multiple smaller networks to be summarized into a single, larger routing entry. This reduces the size of routing tables on internet routers, making routing more efficient and faster.
Flexible Subnetting (VLSM): Variable Length Subnet Masks allow network administrators to divide an IP network into subnets of different sizes, optimizing address usage within an organization.

Routing Aggregation with CIDR

Multiple specific routes can be summarized into a single, more general route.

graph TD subgraph Internet Router A[Route 1: 192.168.1.0/24] --> R B[Route 2: 192.168.2.0/24] --> R C[Route 3: 192.168.3.0/24] --> R D[Route 4: 192.168.4.0/24] --> R end subgraph CIDR Aggregation R(Router) --> S[Aggregated Route: 192.168.0.0/22] end S --> N1(Network 192.168.1.0/24) S --> N2(Network 192.168.2.0/24) S --> N3(Network 192.168.3.0/24) S --> N4(Network 192.168.4.0/24) style A fill:#f0f9ff,stroke:#cbd5e1,stroke-width:2px,color:#475569 style B fill:#f0f9ff,stroke:#cbd5e1,stroke-width:2px,color:#475569 style C fill:#f0f9ff,stroke:#cbd5e1,stroke-width:2px,color:#475569 style D fill:#f0f9ff,stroke:#cbd5e1,stroke-width:2px,color:#475569 style R fill:#7dd3fc,stroke:#0ea5e9,stroke-width:2px,color:#000 style S fill:#0ea5e9,stroke:#0ea5e9,stroke-width:2px,color:#000 style N1 fill:#e0f2fe,stroke:#93c5fd,color:#475569 style N2 fill:#e0f2fe,stroke:#93c5fd,color:#475569 style N3 fill:#e0f2fe,stroke:#93c5fd,color:#475569 style N4 fill:#e0f2fe,stroke:#93c5fd,color:#475569

CIDR Calculator & Visualizer

Enter an IP address and a prefix length to see the network details and how the bits are divided.

IP Address

Prefix Length (/)

Binary Representation

Network Address:

Broadcast Address:

First Usable Host:

Last Usable Host:

Subnet Mask:

Total Hosts: