What is VLSM? how to Calculate VLSM? where it is use and
why?

Answer Posted / mac

Some of the above answers are fully wrong and some half wrong
Here is the correct answer.

VLSM is actually used to keep a tab on number of hosts.
This is used after sub-netting a network.
So first we have to do subnetting and after that we have to
do VLSM for each subnet to keep a tab on number of hosts in
each subnet.

So consider a building with 5 divisions. Subnetting means we
are calculating ip address for each division(also we
calculate the set of ipaddress for hosts for each division).
but in each division it is not really required to have no:of
host that no:of ip address we calculated.
So by doing VLSM on each division(5 times in this case) we
can get a optimised set of ip address for each division.

lets calculate a example VLSM .

Lets say we got a subnet for one of the division as
192.168.0.1 which has 50 hosts.

Bit Value 128 64 32 16 8 4 2 1

Bits Borrowed 1 2 3 4 5 6 7 8

Subnet mask 128 192 224 240 248 252 254 255

Subnet Prefix/CIDR /25 /26 /27 /28 /29 /30




Lets start with 50 hosts, using the table above:
We are borrowing 2 bits with value of 64. This is the
closest we can get for 50 hosts.

- 192.168.1.0 /26 Network address

192.168.1.1 Gateway address

192.168.1.2, First usable address

192.168.1.62- Last usable address. Total address space
-192.168.1.2 to 192.168.1.62

192.168.1.63 will be the broadcast address (remember to
reserve the first and last address for the Network and
Broadcast)

Network Mask 255.255.255.192 - we got the 192 by adding the
bit value from the left to the value we borrowed = 128+64=192
address will look like this 192.168.1.0 /26

Is This Answer Correct ?    21 Yes 2 No



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