This article gives an overview of Variable Length Subnet Masking, How it Works, It’s purpose, Advantages and Disadvantages Of VLSM.
Variable Length Subnet Masking
Within a single IP network, subnets with distinct subnet masks can be created using the network design approach known as variable length subnet masking (VLSM). With this method, network administrators can more effectively and efficiently assign IP addresses by customizing subnet sizes to meet the unique host needs of every network segment.
Variable Length Subnet Masking Purpose and Core Concept
In the past, subnetting entailed giving each subnet in a network a Fixed Length Subnet Masking (FLSM), which frequently resulted in inefficient utilization and IP address waste. For instance, a fixed mask of 255.255.255.0 (offering 254 addresses) would be used for both subnets in a network that required 10 hosts and 50 hosts, squandering a lot of addresses for the smaller subnet.
By allowing the employment of several masks inside a single network, VLSM solves this issue. It enables “subnetting a subnet,” in which more specialized, smaller subnets are formed to precisely match the number of hosts or devices required. With IPv4, where address space is constrained, this is very important.
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How Does VLSM Works
Strategic planning and allocation are necessary for the implementation of VLSM.
Determine Host Needs
Find out how many hosts are needed for each network section. Network and broadcast addresses must be taken into consideration because they are reserved.
Sort by Size: From highest to lowest, arrange all network segments according to their host requirements. This makes it easier to fit all segments without overlapping by guaranteeing that the largest subnets are assigned first.
Pick the Right Masks
For every segment, pick a subnet mask that is both sufficiently big to fit the necessary hosts and not too big. Subnet sizes (e.g., 32, 64, 128 hosts) are expressed in powers of two. Classless Inter-Domain Routing (CIDR) notation, such /24, /26, and /28, is frequently used by VLSM.
Distribute Subnets
- Commence with the most significant prerequisite. Give it the largest IP block that is available.
- Always choose the smallest subnet mask that satisfies the host count when assigning from the remaining IP address space to the next greatest requirements.
- This guarantees less IP waste.
For instance, if you have 256 addresses in your initial IP block (192.168.1.0/24), and you need 100, 50, and 25 hosts:
- Assign 192.168.1.0/25 (128 addresses) to 100 hosts.
- 50 hosts: Use the remaining space to assign 192.168.1.128/26 (64 addresses).
- 25 hosts: Use the remaining space to assign 192.168.1.192/27 (32 addresses).
VLSM Fundamentals
In order to understand VLSM, one must comprehend:
- Subnet Mask: A 32-bit value that distinguishes the network from host components of an IP address.
- Subnetting: By providing access controls and lowering congestion, subnetting divides a big network into smaller, logical or physical subnets to increase security, manageability, and efficiency.
- Supernetting: In order to decrease routing table entries and increase network efficiency, super netting the reverse of subnetting combines several smaller subnets into a single, bigger subnet.
You can also read Fixed Length Subnet Masking Advantages And Disadvantages
Comparison with Fixed Length Subnet Mask (FLSM)
| Feature | Variable Length Subnet Mask (VLSM) | Fixed Length Subnet Mask (FLSM) |
|---|---|---|
| Subnet Size | Subnets can be of different sizes, tailored to the number of hosts required. | All subnets are of equal size. |
| IP Address Utilization | Highly efficient, as it minimizes wasted IP addresses by creating smaller subnets for segments with fewer hosts. | Inefficient, as it assigns the same number of addresses to all subnets, leading to waste if some segments require very few hosts. |
| Subnet Mask | Uses a different subnet mask for each subnet, depending on the number of hosts. | Uses the same subnet mask for all subnets. |
| Network Complexity | More complex to design and implement, requiring careful planning and calculation. | Simpler to design and manage due to the uniform nature of the subnets. |
| Scalability | More scalable and flexible, allowing for future network growth without the need to re-address the entire network. | Less scalable and rigid, making it difficult to add new segments with different host requirements. |
| Routing Protocols | Requires classless routing protocols that support subnet mask information (e.g., OSPF, EIGRP, RIPv2). | Can work with both classful (RIPv1) and classless routing protocols, though less efficient with classful. |
| Application | Ideal for large, complex networks with varying host requirements across different departments or segments. | Best suited for small, simple networks where all segments have similar host requirements. |
Advantages of VLSM
- Efficient IP Address Utilization: This is the main advantage; it keeps IP addresses from being wasted by closely matching subnet sizes to host requirements.
- More Flexibility: Enables network managers to customize IP address allocations to meet the unique requirements of every segment.
- Better Network Management and Scalability: This makes it easier to manage and scale big networks by giving you more control over network architecture. It allows for expansion and modification without necessitating a total redesign.
- Better Network Performance: By establishing smaller broadcast domains, it is possible to lessen network congestion and enhance data flow between subnets.
- Less Network Management Overhead: Makes IP address distribution and administration easier, particularly in big networks.
- Supports Hierarchical Network Design: This technique makes troubleshooting easier by breaking up networks into smaller, easier-to-manage subnets.
Disadvantages of VLSM
- Requirements for routing protocols: Classless routing protocols like RIPv2, OSPF, and EIGRP, which include subnet mask information in their routing updates, are necessary for VLSM to function. VLSM is not supported by older, classful protocols such as RIPv1.
- Complexity increase: Compared to FLSM, VLSM necessitates more meticulous planning and computation. Network issues and overlapping address ranges might result from improperly computed subnets.
Routing Protocol Support
A network has to use a classless routing protocol in order to implement a VLSM design. In order for routers to comprehend different subnet sizes, these protocols incorporate the subnet mask information into their routing updates. The following are typical classless routing protocols that enable VLSM:
- OSPF stands for Open Shortest Path First.
- BGP, or the Border Gateway Protocol
- Router Information Protocol 2
- EIGRP stands for Enhanced Interior Gateway Routing Protocol.
- IS-IS (Intermediate System-to-Intermediate)
VLSM is not supported by older, classful routing protocols like RIPv1 and IGRP because they assume a consistent mask throughout the network and do not send subnet masks in their routing updates.
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