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VLSM Subnetting: Variable Length Subnet Masking for Network Addressing, Summaries of Communication

Variable Length Subnet Masking (VLSM) as an extension of classic subnetting. VLSM allows breaking down subnets into smaller subnets according to network requirements. a step-by-step guide to VLSM subnetting and an example of a company that needs 6 subnets and 160 host addresses.

Typology: Summaries

2021/2022

Uploaded on 09/27/2022

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Computer Networking & Communication 4th Class Arranged By: Dr.Ahmed Chalak Shakir.
Page 1 of 6 (Kirkuk University, College of Science, Computer Science Dept.)
Variable Length Subnet Mask
Classful subnetting:
FLSM (Fixed Length Subnet Mask) is also known as classful subnetting as all subnets have same number
of hosts. In Classful subnetting all subnets use the same subnet mask.
Classless subnetting:
VLSM is also known as classless subnetting as all subnets may have different number of hosts depending
upon network requirement.
VLSM Subnetting:
Variable Length Subnet Mask (VLSM) extends classic subnetting. VLSM is a process of breaking down
subnets into the smaller subnets, according to the need of individual networks. In the following example a
company has requirement of 6 subnets and 160 host addresses. With VSLM you can fulfill this requirement
with single class C address space.
In VLSM Subnetting, we make a subnetting of subnets according to the network requirement.
Steps for VLSM Subnetting:
Find the largest segment. Segment which needs a largest number of hosts addresses.
Do subnetting to fulfill the requirement of largest segment.
Assign the appropriate subnet mask for the largest segment.
For second largest segments, take one of these newly created subnets and apply a different, more
appropriate, subnet mask to it.
Assign the appropriate subnet mask for the second largest segment.
Repeat this process until the last network.
VLSM Example
Now you know the steps of VLSM Subnetting. Let's understand it with example. Our company
requires 6 subnets and 160 hosts.
Step 1 :- Oder all segments according to the hosts requirement (Largest to smallest).
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Variable Length Subnet Mask

Classful subnetting:

FLSM (Fixed Length Subnet Mask) is also known as classful subnetting as all subnets have same number

of hosts. In Classful subnetting all subnets use the same subnet mask.

Classless subnetting:

VLSM is also known as classless subnetting as all subnets may have different number of hosts depending

upon network requirement.

VLSM Subnetting:

Variable Length Subnet Mask (VLSM) extends classic subnetting. VLSM is a process of breaking down

subnets into the smaller subnets, according to the need of individual networks. In the following example a

company has requirement of 6 subnets and 160 host addresses. With VSLM you can fulfill this requirement

with single class C address space.

In VLSM Subnetting, we make a subnetting of subnets according to the network requirement.

Steps for VLSM Subnetting:

 Find the largest segment. Segment which needs a largest number of hosts addresses.  Do subnetting to fulfill the requirement of largest segment.  Assign the appropriate subnet mask for the largest segment.  For second largest segments, take one of these newly created subnets and apply a different, more appropriate, subnet mask to it.  Assign the appropriate subnet mask for the second largest segment.  Repeat this process until the last network.

VLSM Example

Now you know the steps of VLSM Subnetting. Let's understand it with example. Our company

requires 6 subnets and 160 hosts.

Step 1 :- Oder all segments according to the hosts requirement (Largest to smallest).

Step 2 :- Do subnetting for largest segment. Our largest segment needs 74 host addresses. /25 provide us two subnets with 128 hosts in each subnet.

192.168.1.0/

Step 3 :- Assign subnet mask to the largest segment. As you can see in above table, subnet 1 fulfills our largest segment requirement. Assign it to our segment.

Step 4:- Do subnetting for second largest segment from next available subnet. Next segment requires 52 host addresses. Subnetting of /25 has given us two subnets with 128 hosts in each, from that we have assigned first subnet to development segment. Second segment is available; we would do subnetting of this. / provide us 4 subnets with 64 hosts in each subnet.

192.168.1.0/

Subnets 1 to 6 [ address from 0 to 191] are already occupied by previous segments. We can assign subnet 7 to this segment.

Step 6 :- Our last three segments require 2 hosts per subnet. Do subnetting for these.

192.168.1.0/

Valid subnets are:-

From these subnets, subnet 1 to subnet 56 (Address from 0 - 220) are already assigned to previous segments. We can use 224,228, and 232 for wan links.

Assign these subnets to wan links.

Wan Link 1

Wan Link 2

Wan link 3