How to implement VLSM

Variable Length Subnet Mask (VLSM) refers to a subnet design that uses multiple masks within a single network or a single class A, B, or C. It also describes the process of subnetting a subnet.

Note: The starting address of every subnet is called the subnet address, and the ending address is called the broadcast address.

Importance

VLSM provides an effective method to match the address needs with the subnet's requirement. For instance, a mask with fewer host bits for subnets requires fewer addresses, resulting in fewer host IP addresses for the subnet. This approach reduces the number of unused IP addresses in each subnet. Less address wastage makes room for more subnets.

Implementation

Suppose the network administrator has to manage three departments in terms of IP address allocation as follows:

• Academic with 110 devices.
• Accounts with 50 devices.
• Research with 40 devices.

The provided IP address 192.200.10.1/24 belongs to class C.

Only the most significant bit of the host part will be fixed to find the subnetwork addresses of these departments. A table is given below for a better understanding of the solution:

Using the table above to fulfill the requirement of 120 devices for the academic department, we'll only fix the leading first bit of the host part. In this way, we get a total of 126 usable IP addresses. The illustration is given below:

For the accounts department, we'll fix 2 bits. In the end, we get 62 usable host IP addresses.

As only 40 devices will be used in the research department, the most efficient subnet will be the one having 2 fixed bits. Consequently, we'll get 62 usable host IP addresses.

Features

The salient features of VLSM are discussed below:

• It provides simple network configuration.
• It has higher efficiency compared to fixed-length subnet masks (FLSM).
• It uses simplified routingA type of routing in which a router only uses a VLSM sequence and not a full IP address to communicate within the network..

Applications

VLSM has a wide range of applications in network systems and services, such as the following:A type of routing in which a router just uses a VLSM sequence and not a full IP address to communicate within the network

• Border Gateway Protocol (BGP).
• Enhanced Interior Gateway Routing Protocol (EIGRP).
• Intermediate System to Intermediate System Protocol (IS-IS).
• Cisco's Open Shortest Path First (OSPF).