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).
