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Module - 4 VLSM - Variable Length Subnet Mask

2026-09-05 09:40

Tags: #Network

Author: Duke Hsu

Topic

  1. VLSM (Variable Length Subnet Mask)
  2. Step by Step

1. VLSM

VLSM stands for Variable Length Subnet Mask.

  • You can use more than one mask inside the same Class A/B/C network .

  • You can create subnets of different sizes based on actual requirements.

  • It is often described as subnetting the subnet.

2. VLSM Step by step

VLSM-Step-by-Step.png

2.1Step 1 : List all networks and their hosts requirement

Find subnet size - Host needs - CIDR / Prefix - Subnet Mask

For example

4000 hosts

\(2^{11} - 2 =2044\) not enough \(2^{12}-2=4094\) ok > 4000

Host bits = 12

Network bits = 32 - 12 = 20 CIDR : /20

Subnet mask = 255.255.240.0

2.2 Step 2 Arrange the requirements from largest to smallest

Find network address

  • Interesting Octet
  • Increment number / Magical number
  • Network Address

For example

CIDR = /20

255 . 255 . 240 . 0

Interesting Octet = Octet 3

Interment number = 256 - subnet mask value 

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
ST
256 - 240 = 16

CT
256 - 240 = 16

SV
256 - 248 = 8

CS
256 - 252 = 4

2.3 Step 3 - Determine the appropriate subnet mask for each requirement.

Find Address Range

  • Broadcast IP
  • 1st Usable IP
  • Last Usable IP

Network + 1 = First Broadcast - 1 = Last

Notice

0 ~ 7 is 8 bits , included 0, 1, 2, 3, 4, 5 , 6 , 7

References

https://www.youtube.com/watch?v=P5t2gN2l7ZI