CIDR (Classless Inter-Domain Routing) and VLSM (Variable Length Subnet Masking)

CIDR (Classless Inter-Domain Routing) and VLSM (Variable Length Subnet Masking)

In the domain of networking, two pivotal concepts revolutionized the way we allocate and manage IP addresses: CIDR (Classless Inter-Domain Routing) and VLSM (Variable Length Subnet Masking). CIDR emerged as a response to the limitations of traditional IP addressing, introducing a more flexible and efficient approach to representing IP address ranges. On the other hand, VLSM, a crucial extension of CIDR, empowers network administrators with the ability to optimize subnetting by allowing variable-length subnet masks. Together, CIDR and VLSM have become indispensable tools in modern networking, enabling streamlined IP address allocation, efficient routing, and scalability in diverse network architectures. This tutorial will guide you through the intricacies of CIDR and VLSM, providing a comprehensive understanding and practical insights into their applications. You can also visit here the a detailed tutorial on networking. You can also visit the detailed tutorial on computer networks here.

CIDR (Classless Inter-Domain Routing)

Traditional IP addressing faced limitations with class-based systems. CIDR, or Classless Inter-Domain Routing, emerged as a solution to overcome these limitations, allowing for more efficient allocation of IP addresses. CIDR addresses the growing need for a scalable IP addressing system. Without CIDR, IP address exhaustion becomes a significant concern. We’ll explore real-world examples illustrating the necessity of CIDR.

CIDR Notation

CIDR notation involves breaking down an IP address into two components: the network address and the subnet mask. We’ll delve into the significance of each component, emphasizing the use of the forward slash (/) in CIDR notation.

Suppose we have the IP address range to In traditional IP addressing, this might be represented as a Class C network with a subnet mask of However, with CIDR notation, we can represent this range more efficiently.

The CIDR notation for this range would be

Breaking it down:

  • This is the network address.
  • /24: This indicates that the first 24 bits are used for the network portion of the address, leaving 8 bits for host addresses.

So, in CIDR notation, the range to is encompassed by the single notation

This CIDR representation not only provides a concise way of expressing the range but also allows for efficient routing and allocation of IP addresses. It’s a powerful tool in modern networking, especially when dealing with diverse IP address ranges.

VLSM (Variable Length Subnet Masking)

Variable Length Subnet Masking (VLSM) stands as a pivotal advancement in the realm of networking, introducing a level of flexibility and efficiency that transforms how we design and manage subnets. In traditional subnetting, subnet masks remain fixed in length across an entire network. However, VLSM allows network administrators to break free from this constraint by permitting the use of different subnet mask lengths within the same network. This innovation is particularly beneficial in scenarios where subnets vary in size, as it enables the optimization of IP address space allocation. With VLSM, networks can be tailored to specific needs, fostering a more efficient utilization of resources and enhancing the scalability of intricate network architectures.

Suppose we have a network with the IP address and we need to subnet it to accommodate three different subnets of varying sizes.

  1. Subnet A: Requires 60 host addresses.
  2. Subnet B: Needs to support 30 host addresses.
  3. Subnet C: Only needs 10 host addresses.

For Subnet A, we can use a subnet mask of /26, which provides 64 addresses, accommodating the required 60 hosts. So, the subnet would be

For Subnet B, we can use a subnet mask of /27, providing 32 addresses. This allows us to allocate the needed 30 host addresses. The subnet would be

For Subnet C, we can use a subnet mask of /28, offering 16 addresses. This provides more than enough for the 10 hosts required. The subnet would be

In this example, VLSM enables us to customize the subnet mask length for each subnet, optimizing the allocation of IP addresses based on the specific needs of each network segment. VLSM’s flexibility allows for efficient utilization of IP address space, contributing to the scalability and adaptability of network designs.

Benefits of CIDR and VLSM

CIDR (Classless Inter-Domain Routing) and VLSM (Variable Length Subnet Masking) offer substantial benefits in the realm of IP address management and network design.

CIDR revolutionized IP address allocation by introducing a more flexible approach compared to the traditional class-based system (Class A, B, C). With CIDR, IP addresses are allocated in variable-length subnet masks (VLSM), allowing networks to use only the required number of addresses per subnet. This efficient allocation reduces IP address wastage, especially crucial given the global scarcity of IPv4 addresses. CIDR also optimizes routing by enabling the aggregation of IP routes into larger blocks. This aggregation reduces the size of routing tables, making internet routing more efficient and scalable.

VLSM, as an extension of CIDR, enhances the optimal utilization of IP address space within a network. By allowing different subnets to have subnet masks of varying lengths, VLSM enables the creation of subnets tailored to specific needs. This flexibility supports a more efficient network design, where subnet sizes can be adjusted according to the number of hosts required per subnet. This approach not only conserves IP addresses but also simplifies network management by organizing IP address allocation in a hierarchical and structured manner.


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Self Evaluation

  • Outline the key problem that CIDR addresses in traditional IP addressing. How does CIDR contribute to more efficient global routing on the Internet? Provide a real-world example to illustrate its significance.
  • Explain the fundamental concept of Variable Length Subnet Masking (VLSM) and how it differs from fixed-length subnetting. Provide an example scenario where VLSM would be particularly advantageous.
  • You are tasked with designing a network that requires three subnets: one for 60 hosts, one for 30 hosts, and one for 15 hosts. Demonstrate how you would implement VLSM to efficiently allocate IP addresses for each subnet. Provide the subnet addresses and subnet masks.

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