What’s CIDR? Subnets and CIDR Defined

Classless Inter-Area Routing (CIDR) is an IP addressing system that changed the earlier system of utilizing Class A, B, and C networks. CIDR was developed to assist alleviate the scarcity of obtainable IP addresses by permitting extra environment friendly tackle allocation utilizing extra distinctive and granular identifiers for networks and particular person units.

To interrupt that down into a less complicated clarification, consider IP addresses as house addresses. Similar to how you may have a home tackle to obtain mail, computer systems want IP addresses to speak with one another.

Earlier than CIDR, IP addresses had been assigned in teams referred to as courses. It was like having a home tackle that was solely 3 digits lengthy. This meant that there have been a really restricted variety of addresses accessible.

CIDR is like having a home tackle that’s 10 digits lengthy. Which means there are extra home addresses accessible and they are often assigned with extra effectivity and specificity.

What’s the aim of CIDR?

In 1993, CIDR was launched as an answer to the looming shortage of IPv4 addresses. By allotting to networks extra exact and nuanced CIDR blocks that eat fewer assets, CIDR has been capable of prolong the lifespan of this treasured useful resource. It changed the now out of date classful community system based mostly on courses A, B, and C as summarized in IETF’s rfc943:

Classful network system demonstrating class A, B, and C addresses.
Determine A: Classful Community Addressing

Disadvantages of the classful system

The classful system, whereas nicely suited to its time, had drawbacks that outdated it rapidly, together with inefficiency, poor construction, routing points, and lack of scalability.

1. Inefficiency

The classful system was inefficient by way of the variety of addresses it allotted. It was additionally rigid: with solely three sizes accessible, it didn’t enable networks to be simply modified or cut up up. 

CIDR solves these points by making it simpler to divide and allocate IP addresses extra effectively.

2. Structuring

When CIDR is used, an IP tackle might comprise a prefix which describes the community and a suffix which describes a selected gadget or host on that community. 

CIDR additionally permits for CIDR blocks, that are outlined as a sure variety of IP addresses that may be allotted to a single group. 

Classful IP addresses didn’t enable both of those choices.

3. Routing points

The classful system necessitated that every impartial community be listed by itself, forming bigger routing tables. 

To fight this drawback, CIDR devised a notation plan to group networks collectively, considerably decreasing the entries on these giant routing tables.

4. Lack of scalability

Quickly after the introduction of the Area Title System (DNS), the classful system rapidly turned untenable due to the sheer dimension and scale of the web. 

CIDR solved this drawback by permitting particular CIDR blocks to be allotted effectively all through the web, bettering scalability.

How CIDR works

By using CIDR notation, directors can mixture networks in a method that minimizes the variety of entries in a routing desk. This technique entails assigning CIDR blocks to organizations and subdividing these blocks into smaller, extra manageable segments. Because of this, IP addresses might be dynamically allotted to customers based mostly on particular guidelines and necessities.

The Web Assigned Quantity Authority (IANA) shoulders the accountability of managing CIDR block assignments. On this method, CIDR streamlines the method of allocating IP addresses dynamically based on pre-established pointers, making certain a extra organized and environment friendly system for community administration.

CIDR structuring

CIDR is structured utilizing blocks containing prefixes, suffixes, and route notation.


CIDR blocks are outlined as a sure variety of IP addresses that may be allotted to a single group. CIDR blocks are divided into smaller chunks, which makes it simpler to divide and allocate IP addresses extra effectively.


CIDR blocks are denoted by the prefix and suffix. The prefix signifies the community and the suffix signifies a selected host inside that community.


CIDR notation pairs an IP tackle with a particular community masks. As an example, contemplate the next format:—the place 17 displays what number of bits are within the tackle itself. 

IPv4 addresses can embrace as much as 32 of those bits, whereas IPv6 is extra intensive and permits 128 doable bit combos. Put merely, CIDR supplies you with most management over routes between your computer systems and networks.

Subnet masks

CIDR is an efficiency-oriented system rooted within the idea of subnet masks. These 32-bit numbers act as a filter for IP addresses, logically partitioning them into community and host parts that allow units to rapidly discern how finest to speak with each other. With this environment friendly division of parts, information might be swiftly transferred between hosts with out confusion or delay.

CIDR is an alternate technique of representing a subnet masks and is made up of the binary illustration of the community tackle adopted by a slash and a suffix that declares the entire variety of bits reserved that should be matched within the tackle.

What does a CIDR tackle appear like?

A CIDR tackle consists of two components: the prefix and the suffix. The prefix is the community tackle. It’s adopted by a slash and the suffix which denotes the CIDR block dimension in bits. 

For instance, you probably have an IP tackle of and a subnet masks of, the CIDR notation could be The /24 signifies that the primary 24 bits of the IP tackle are used for the community tackle, and the remaining 8 bits are used for the host tackle.

The CIDR tackle might be written in IPv4 or IPv6 notation. For instance for IPv4, or 2001:db8::/32 for IPv6.

2 examples of CIDR conversion

Changing CIDR addresses to IP addresses and vice versa is kind of easy; nonetheless, sure guidelines should be adopted when doing so. For instance, CIDR notation should have a suffix (the CIDR block dimension) between 0 and 32 for IPv4 addresses and 0 to 128 for IPv6 addresses.

There are two main technique of changing IP addresses to CIDR: subnetting and supernetting.


Subnetting is the method of dividing a community into smaller subnetworks to enhance community efficiency and safety. 

Subnetting is completed by borrowing bits from the host ID portion of the IP tackle. For instance, you probably have a Class C community tackle of and also you wish to create 4 subnets, you’d borrow 2 bits from the host ID portion of the IP tackle. This may provide you with 4 subnets, every with 64 hosts.


Supernetting, in any other case referred to as route summarization or route aggregation, is a technique of condensing quite a few contiguous subnets into one giant community. This course of is accomplished by fastidiously choosing bits from the IP tackle community ID portion to generate an aggregated illustration. 

For instance, you probably have 8 subnets, you may summarize them right into a single subnet by borrowing 3 bits from the community ID portion of the IP tackle. This may provide you with a single subnet with 8 subnets, every with 256 hosts. 

An instance of a sensible utility of this might be when an organization needs to show a number of workplace areas with corresponding routers into one tremendous community.

Backside line: Utilizing CIDR in enterprise networks

CIDR’s authentic function was to control the expansion of routing tables on routers world wide and scale back their dimension. It has exceeded expectations. Along with minimizing router desk sizes, it has helped enhance safety and improve scalability and efficiency. Lengthy after its preliminary intent was fulfilled, CIDR stays an indispensable instrument for community directors right this moment.

Hold your company community in peak efficiency with one of many finest enterprise Wi-Fi answer suppliers on right this moment’s market.


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