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VLSM (Variable Length Subnet Masking)

Introduction

The rapid expansion of networks, coupled with the increasing demand for IP addresses, led to the development of techniques to optimize IP address space utilization. One such crucial method is Variable Length Subnet Masking (VLSM). In this article, we will delve into the fundamentals of VLSM, its advantages, and practical applications.

Understanding the Need for VLSM

The traditional IP address allocation faced challenges as the Internet grew exponentially. The rigid structure of classes (A, B, C) resulted in inefficient use of IP addresses. The exhaustion of the Class B network address space and the strain on routing tables prompted the need for a more flexible solution.

The Birth of VLSM

The Routing and Addressing (ROAD) group, initiated by the Internet Engineering Task Force (IETF), identified three major issues leading to the development of VLSM:

  1. Exhaustion of the Class B network address space.
  2. Growth in internet routers’ routing tables beyond current hardware and software capacities.
  3. Eventual exhaustion of the 32-bit IPv4 network address space.

To address these concerns in the short to mid-term, the ROAD group proposed a solution that allowed the use of “classless” IP assignment systems. This gave rise to Classless Inter-Domain Routing (CIDR) and VLSM, enabling greater flexibility in creating sub-networks.

What is VLSM: Fundamentals

To comprehend what is a VLSM, one must understand subnetting, supernetting, and the concept of subnet masks. A subnet mask is a 32-bit sequence of ones (1) followed by a block of zeros (0). Subnetting involves dividing a large network into smaller subnets to enhance efficiency, while supernetting consolidates multiple networks into a larger one.

Advantages of VLSM

VLSM stands out as a subnet design strategy by allowing subnet masks to have variable sizes. Unlike Fixed Length Subnet Masks (FLSM), where all subnets are of equal size, VLSM permits network administrators to allocate subnets of different sizes based on individual needs. This leads to a more efficient use of IP address ranges.

Implementing VLSM

Let’s explore a practical scenario to understand the implementation of VLSM. Consider a company with multiple departments. By employing VLSM with a given IP range, a network engineer can design an IP plan that efficiently allocates addresses to different subnets based on the size of each department’s network.

Conclusion

VLSM has proven to be a critical technique in modern network design, offering flexibility and efficient IP address utilization. As we move forward, understanding and mastering VLSM becomes imperative for network administrators to design scalable and resource-efficient networks.

Future Perspectives

While VLSM has been instrumental in addressing IPv4 limitations, the transition to IPv6, with its 128-bit address space, provides a more sustainable solution in the long run.

In summary, VLSM continues to be a cornerstone in the evolution of networking, ensuring that IP addresses are used judiciously in the face of ever-growing connectivity demands.

Alexey Shkittin

CEO

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