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Difference between CIDR and VLSM

The internet's exponential growth over the last 20 years exposed flaws in the original IP protocol. CIDR and VLSM provide additional flexibility in creating sub-networks. The terms CIDR and VLSM are expressly utilized to create a network. CIDR is utilized for combining routes to reduce the routing data carried by the core routers. On the other hand, VLSM aids in optimizing the available address space. CIDR is the inverse of VLSM in that it defines rules for referencing a group of networks using a single route statement. On the other hand, VLSM specifies rules for subdividing networks.

In this article, you will learn about the difference between CIDR and VLSM. But before discussing the differences, you must know about CIDR and VLSM with their advantages and disadvantages.

What is CIDR?

CIDR is an abbreviation for "Classless Inter-Domain Routing". It is a collection of Internet Protocol standards that are utilized to generate unique IDs for networks and single devices. IP addresses allow specific data packets to be sent to specified computers. It is an IP address assignment technique that enhances address distribution efficiency. It is also referred to as supernetting and replaces the previous approach based on classes A, B, and C networks. Several unique IP addresses may be defined utilizing a single CIDR IP address. CIDR IP addresses are the same as regular IP addresses, except they end with a slash that is followed by a number.

When compared to classful addressing, there is no waste of IP addresses in CIDR because only the number of IP addresses requested by the user is assigned to the user. The collection of IP addresses is known as Block in CIDR.

CIDR follows both CIDR notation and Slash notation. CIDR notation is represented as x.y.z.w /n, and the x.y.z.w is an IP address, and n is the mask or number of bits that are utilized in network id.

How does CIDR Works?

CIDR IP addresses are composed of two groups of numbers, often referred to as groups of bits. The network address is the most essential of these groupings and is utilized to define a network or sub-network (subnet). Some of the bit groups are the host identifier, and the host identifier is utilized to define which host or device on the network must be receiving incoming data packets.

Unlike classful routing, which categorizes addresses into one of three blocks, CIDR permits internet service providers (ISPs) to be assigned blocks of IP addresses. The blocks are subsequently divided and assigned to consumers of the supplier. In IPv6, a CIDR block is usually given 64 bits to describe network addresses.

Characteristics of CIDR

It dynamically assigns IP addresses based on the user's requirements utilizing the CIDR blocks and certain rules. The Internet Assigned Numbers Authority (IANA) assigns the CIDR block. The CIDR block has IP addresses and several rules. Some rules of CIDR are as follows:

  1. All IP addresses assigned to a host should be continuous.
  2. The block size should be power 2 and equal to the total IP addresses.
  3. The block's size should be divisible by the block's first IP address.

Advantages and Disadvantages of CIDR

There are various advantages and disadvantages of CIDR. Some advantages and disadvantages of CIDR are as follows:


  1. It permits efficient routing entries, meaning that a small number of routing entries may describe many networks.
  2. It offers efficient address space allocation, which means that addresses may be assigned in sizes of any binary multiple.
  3. It compressed the network description and described it by a single entry.
  4. It removes the class imbalance, which means that there are no more class networks, allowing you to utilize certain address spaces while ignoring others broadly.
  5. There is no need for a different subnetting method because it incorporates the idea of subnetting within the internet itself.
  6. It produces reduced overhead in terms of network traffic, processor, and memory.


  1. It's no longer possible to define how many bits of an IP address indicate the network ID and how many reflect the host ID by looking at the first octet.
  2. Its main drawback is its complexity.

What is VLSM?

VLSM is an abbreviation for "Variable Length Subnet Mask". It is a mechanism for allocating host space of different sizes between networks by dividing a network into numerous subnetworks. It was primarily created to provide greater flexibility for creating a network using several masks.

In other terms, it is a mechanism for applying numerous subnet masks to a given class of addresses over a routed system. Previously, it was not possible because they (such as RIPv1protocol) didn't support the subnet mask of listed networks in their routing updates. As a result, they cannot learn the existence of multiple mask lengths.

Classless routing protocols such as OSPF, IS-IS, RIPv2, EIGRP, and BGP enable VLSM implementation by integrating the subnet mask for the networks announced in routing updates. VLSM also supports several subnet masks inside the same network address space, which is known as "subnetting a subnet". Previously, network utilization was restricted to only/26 masks throughout the system.

How does VLSM work?

Routing protocols have always necessitated that a single network utilizes the same subnet mask. VLSM permits networks to have various subnet masks if the routing protocol used on the network supports it. VLSM also deviates from convention by using the first and last subnets, which were generally reserved to avoid confusion when the network and subnet had the same address. When it is completed, VLSM offers eight useable subnets, each of which may serve 30 hosts.

Implementation of VLSM

In VLSM, subnets utilize block size based on the need, so subnetting is needed multiple times. Suppose an organization has four Classes with different needs, including Class A needs 110 hosts, Class B needs 55, Class C needs 25, and Class needs 5.

Class A: 110 hosts

Class B: 55 hosts

Class C: 25 hosts

Class D: 5 hosts

If an organization has IP, Class wise IPs may be assigned by utilizing the following these steps:

Step 1: Choose a block size for every segment. This number should be larger than or equal to the total of the host, broadcast, and network addresses. List all possible subnets:

Host/Subnets Subnet Mask
254 /24
126 /25
62 /26
30 /27
14 /28
6 /29
2 /30

Step 2: Arrange all of the segments in descending order based on the block size needed, from highest to lowest.

  1. Class A: 110 hosts
  2. Class B: 55 hosts
  3. Class C: 25 hosts
  4. Class D: 5 hosts

Step 3: Assign the suitable subnet mask to every subnet. Determine the highest IP that is available and assign it to the highest need. Therefore, there are have 126 valid IP addresses that may be assigned to the 110 hosts needed by Class A. The subnet mask utilized is

Step 4: The next phase necessitates an IP address capable of supporting 55 hosts. The IP subnet with network number is the next highest and may be allocated to 62 hosts, fulfilling the development department's needs. The subnet mask utilized is

Step 5: Similarly, the next IP subnet may meet the needs of Class C because it includes 30 legitimate hosts' IP addresses that may be allocated to 25 hosts. The mask utilized is

Step 6: The final section needs 5 legitimate hosts' IP addresses, which may be fulfilled by utilizing the subnet with the mask The IP with the mask may be selected, but it contains 14 valid host IPs, and the need is less, so the equivalent one is selected.

Advantages and Disadvantages of VLSM

There are various advantages and disadvantages of VLSM. Some advantages and disadvantages of VLSM are as follows:


  1. The efficient use of the address space.
  2. Hierarchical addressing is possible in VLSM.
  3. There is very little IP address wastage.
  4. A single "summary" address may be shared by many networks, which minimizes the size of the routing table and speeds up route lookups.


  1. Implementing VLSM may be more difficult than utilizing subnets of all the same size.

Key differences between CIDR and VLSM

Difference between CIDR and VLSM

There are various key differences between CIDR and VLSM. Some of the key differences between CIDR and VLSM are as follows:

  1. CIDR employs suppernetting, which refers to the aggregation of the network into a single address. On the other hand, VLSM utilizes the idea of subnetting, which is nothing more than the partition of a single network into many subaddresses.
  2. The CIDR is a summary of the subnets back to the classes. On the other hand, VLSM allows you to employ variable subnet masks to the same class address space.
  3. BGP and OSPF are two protocols that help CIDR. On the other hand, VLSM is implemented using IGRP, RIPv2, OSPF, and BGP.

Head-to-head comparison between CIDR and VLSM

Here, you will learn head-to-head comparisons between CIDR and VLSM. The main differences between CIDR and VLSM are as follows:

Features CIDR VLSM
Full Form CIDR is an abbreviation for "Classless Inter-Domain Routing". VLSM is an abbreviation for "Variable Length Subnet Mask".
Basic It allows routers to combine routes together. It helps in the optimization of available address space.
Concept Utilization It utilizes the concept of supernetting. It utilizes the concept of subnetting.
Supported Protocol BGP and OSPF are two protocols that help CIDR. IGRP, RIPv2, OSPF, and BGP protocols that support VLSM.


The CIDR enables the aggregation of many networks into a single address, which is accomplished through the use of a routing table entry that describes the network aggregation. On the other hand, the VLSM aids in the creation of a hierarchy of subnets of varying sizes from an IP address space.

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