Understanding CIDR: A Comprehensive Guide

What is CIDR?

CIDR (Classless Inter-Domain Routing) is an improved method for allocating IP addresses and routing data on the internet. It replaced the older system to make internet communications more efficient.

Why is CIDR Important?

Every device connected to the internet needs a unique address, called an IP address. These addresses allow devices to communicate with each other. CIDR helps manage these addresses better than older methods.

Understanding IP Addresses

IPv4 Addresses

An IPv4 address consists of 32 bits, usually shown as four numbers separated by periods. Each number can range from 0 to 255.

Example: 192.23.12.3 is an IPv4 address

The Old Way vs. CIDR

The Old Way (Classful IP Addressing)

Organizations would buy IP addresses in three fixed classes:

• Class A: 8 bits for network, 24 for hosts (e.g., 44.0.0.1)
• Class B: 16 bits for network, 16 for hosts (e.g., 128.16.0.2)
• Class C: 24 bits for network, 8 for hosts (e.g., 192.168.1.100)

Limitations of the Old Way

• Class A supported 16,777,214 hosts
• Class B supported 65,534 hosts
• Class C supported 254 hosts

This system often led to wasted IP addresses. For example, if you needed 300 IP addresses, you'd have to buy a Class B block, wasting over 65,000 addresses.

It also made network design inflexible. You couldn't easily combine addresses from different classes into a single network.

The New Way (CIDR)

CIDR allows for more flexible sizing of network addresses. It uses a notation like 192.168.1.0/24, where the number after the slash indicates how many bits are used for the network part of the address.

How CIDR Works

CIDR notation shows how many bits of an IP address belong to the network. For example:

• In 192.168.1.0/24, the first 24 bits are for the network, leaving 8 bits for individual devices.

Calculating Available Addresses

To find out how many addresses are in a CIDR block, use this formula:

Number of addresses = 2^(32 - CIDR prefix length)

Example: For 192.168.1.0/24

• Number of addresses = 2^(32 - 24) = 2^8 = 256

This means you have 256 IP addresses, from 192.168.1.0 to 192.168.1.255.

Benefits of CIDR

1. Efficient IP Address Use: Allocate only the addresses you need.
2. Improved Routing: Helps routers work more efficiently, improving internet speed.
3. Flexibility: Create custom-sized networks to fit your needs.
4. Scalability: Easily grow networks as organizations expand.

Real-World Examples

1. Home or Small Office:

   • Might use: 192.168.1.0/24
   • Provides 256 addresses (254 usable)

2. Large Company:

   • Might use: 10.0.0.0/16
   • Provides 65,536 addresses

3. Part of a Data Center:

   • Might use: 172.16.0.0/20
   • Provides 4,096 addresses

In Simple Terms

Think of CIDR like a flexible apartment numbering system. Instead of having fixed-size buildings with 100 apartments each, CIDR allows for buildings with exactly the number of apartments needed - no waste and easy to expand!