====== CIDR Notation ======

**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:

<code>
192.168.1.25/24
</code>

Here:

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

-----

===== Why Was CIDR Introduced? =====

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.

-----

===== Understanding the Prefix Length =====

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

Example:

<code>
192.168.1.25/24
</code>

means:

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

Visually:

<code>
IP Address

192.168.1.25

↓

11000000.10101000.00000001.00011001

Network Bits

11111111.11111111.11111111

Host Bits

00000000
</code>

-----

===== CIDR and Subnet Masks =====

CIDR notation and subnet masks describe exactly the same thing.

^ CIDR ^ Subnet Mask ^
| /8 | 255.0.0.0 |
| /16 | 255.255.0.0 |
| /24 | 255.255.255.0 |
| /25 | 255.255.255.128 |
| /26 | 255.255.255.192 |
| /27 | 255.255.255.224 |
| /28 | 255.255.255.240 |
| /29 | 255.255.255.248 |
| /30 | 255.255.255.252 |
| /32 | 255.255.255.255 |

CIDR notation is shorter and is therefore preferred today.

-----

===== Examples =====

Example 1:

<code>
192.168.1.0/24
</code>

This represents one subnet containing:

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

-----

Example 2:

<code>
10.0.0.0/8
</code>

A very large network containing over 16 million IPv4 addresses.

-----

Example 3:

<code>
203.0.113.17/32
</code>

Represents exactly one IPv4 address.

-----

===== CIDR in IPv6 =====

CIDR is also used with IPv6.

Example:

<code>
2001:db8::/32
</code>

Most local IPv6 networks use:

<code>
/64
</code>

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

-----

===== Why Is CIDR Useful? =====

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

-----

===== CIDR and Routing =====

Routers use CIDR prefixes when selecting routes.

Example:

<code>
Destination

192.168.10.25

Routing Table

192.168.0.0/16
192.168.10.0/24
0.0.0.0/0
</code>

The router chooses the **longest matching prefix**.

In this example:

<code>
192.168.10.0/24
</code>

is selected because it is more specific than:

<code>
192.168.0.0/16
</code>

This is known as the **Longest Prefix Match** rule.

-----

===== CIDR and Telenegar =====

Several Telenegar tools display or use CIDR notation.

  * [[tools:ipcheck|IP Check]]
  * [[tools:subnetcalc|Subnet Calculator]]

Subnet Calculator can convert between:

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

-----

===== Common Misconceptions =====

  * **/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.

-----

===== Related Topics =====

  * [[networking:internet:ip-addresses|IP Addresses]]
  * [[networking:internet:ipv4|IPv4]]
  * [[networking:internet:ipv6|IPv6]]
  * [[networking:internet:subnetting|Subnetting]]
  * [[tools:subnetcalc|Subnet Calculator]]

-----

{{tag>cidr subnet subnetting ipv4 ipv6 routing networking}}