CIDR (Classless Inter-Domain Routing) is a compact way of describing an IP network.

Instead of using a subnet mask, CIDR specifies how many bits belong to the network portion of an IP address.

Example:

192.168.1.25/24

Here:

• `192.168.1.25` is the IP address.
• `/24` is the CIDR prefix length.

Originally, IPv4 used fixed address classes (Class A, B, and C).

This approach wasted many IP addresses and made routing inefficient.

CIDR was introduced to:

• Improve address allocation
• Reduce routing table size
• Replace classful addressing
• Support flexible subnet sizes

Today, CIDR is the standard method for describing IP networks.

The number after the slash indicates how many bits belong to the network.

Example:

192.168.1.25/24

means:

• First 24 bits → Network
• Remaining 8 bits → Host

Visually:

IP Address

192.168.1.25

↓

11000000.10101000.00000001.00011001

Network Bits

11111111.11111111.11111111

Host Bits

00000000

CIDR notation and subnet masks describe exactly the same thing.

CIDR notation is shorter and is therefore preferred today.

Example 1:

192.168.1.0/24

This represents one subnet containing:

• Network address
• Host addresses
• Broadcast address (IPv4)

Example 2:

10.0.0.0/8

A very large network containing over 16 million IPv4 addresses.

Example 3:

203.0.113.17/32

Represents exactly one IPv4 address.

CIDR is also used with IPv6.

Example:

2001:db8::/32

Most local IPv6 networks use:

/64

Unlike IPv4, IPv6 almost always uses prefix notation rather than subnet masks.

CIDR allows networks of almost any size.

Instead of allocating only fixed-size networks, administrators can choose the most appropriate prefix.

Benefits include:

• Better utilization of IP addresses
• Smaller routing tables
• Flexible subnet sizes
• Simpler network management

Routers use CIDR prefixes when selecting routes.

Example:

Destination

192.168.10.25

Routing Table

192.168.0.0/16
192.168.10.0/24
0.0.0.0/0

The router chooses the longest matching prefix.

In this example:

192.168.10.0/24

is selected because it is more specific than:

192.168.0.0/16

This is known as the Longest Prefix Match rule.

Several Telenegar tools display or use CIDR notation.

• IP Check
• Subnet Calculator

Subnet Calculator can convert between:

• CIDR notation
• Subnet mask
• Network address
• Host range

• /24 means 24 computers.
  • False. It means 24 network bits.

• CIDR is only used with IPv4.
  • False. IPv6 also uses CIDR notation.

• CIDR and subnet masks are different technologies.
  • False. They represent the same information in different formats.

• IP Addresses
• IPv4
• IPv6
• Subnetting
• Subnet Calculator

cidr subnet subnetting ipv4 ipv6 routing networking