Variable Length Subnet Masks (VLSMs)

A VLSM is a process of allocating IP addresses to the subnets according to the user’s need.
VLSM is introduced to avoid the wastage of IP addresses.
It allows you to divide big networks into smaller, different size networks.
It allows an organization to use more than one subnet mask within the same network address space.

Features of VLSMs

A list of features of VLSMs include –

Simple network configuration.
Network IP addressing through empty subnet filling.
Greater efficiency as compare to Fixed Length Subnet Masks (FLSMs) Subnetting.
Reduce the size of the routing table.

There are following routing protocols and services used by the VLSMs –

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

Implementing VLSM Networks

Example:

An administrator has given IP space is 192.168.1.0/24. The slash 24 represents the number of bits used for a network address. The administrator has three different departments with a different number of computers. The development department has 120 computers, Production department has 57 computers, and Accounts has 28 computers. All departments are connected to each other via WAN links.

Show how VLSM can be used in order to allocate department-wise IP addresses in the above example:

Solution:

Step 1: Select the appropriate block size of each item.

Below table shows the list of possible block sizes for the above example:

Prefix	Subnet Mask	Hosts	Block Size
/24	255.255.255.0	254	256
/25	255.255.255.128	126	128
/26	255.255.255.192	62	64
/27	255.255.255.224	30	32
/28	255.255.255.240	14	16
/29	255.255.255.248	6	8
/30	255.255.255.252	2	4

Step 2: Sort the requirement of IPs in descending order.

Development Department = 120

Production Department = 57

Accounts = 28

Step 3: Allocate the highest range of IPs to the highest requirement, so assign 192.168.1.0/25 (255.255.255.128) to the Development department. This IP subnet has 126 valid host IP addresses which can be easily assigned to all the computers of the Development department.

Step 4: Allocate the next highest range of IPs to the next highest requirement, i.e., Production department, so assign 192.168.1.0/26 (255.255.255.192) to the Production department. This IP subnet has 62 valid host IP addresses which can be easily assigned to all the computers of the Production department.

Step 5: Allocate the next highest range of IPs to the next most top requirement, i.e., Accounts, so assign 192.168.1.0/27 (255.255.255.224) to the Production department. This IP subnet has 30 valid host IP addresses which can be easily assigned to all the computers of the Accounts Department.

With the help of VLSM, the administrator can subnet the IP subnet in such a way that the least number of IP addresses are wasted.

Difference between FLSM Subnetting and VLSM Subnetting

FLSM (Fixed Length Subnet Masks) Subnetting	VLSM (Variable Length Subnet Masks) Subnetting
All subnets are equal in size.	Subnets are different in size.
All subnets use the same subnet masks.	All subnets use different subnet masks.
All subnets have an equal number of hosts.	Subnets have a different number of hosts.
It wastages a lot of IP addresses.	It wastages minimum IP addresses.
It is also known as classfull Subnetting.	It is also known as classless subnetting.
It supports both classfull and classless routing protocols.	It only supports classless routing protocols.

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