CCNA 42

VLSM stands for Variable-Length Subnet Mask. VLSM simply means that you have different subnet masks for different subnets. For this article, we'll use the following topology:


Now, suppose we're given a problem like this: Subnet 192.168.1.0/24 to address this network using the most efficient addressing possible.

Now, if we simply divide this network into 8 subnets (to cater for the 6 in the topology), we'll have 32 hosts per subnet and the network with 60 hosts would not fit. To do this, we'll need to give more addresses to those that need it, and less addresses to those that doesn't need it. We'll first need to note down all the subnets and sort them in descending order like this:
60
20
20
2
2
2

Now, we'll write down the number of host bits required for each subnet (Bits Required = RoundUp(Log(x+2)/Log2)):
60 - 6
20 - 5
20 - 5
2 - 2
2 - 2
2 - 2

Now we calculate the CIDR for each subnet:
60 - 6 - /26
20 - 5 - /27
20 - 5 - /27
2 - 2 - /30
2 - 2 - /30
2 - 2 - /30

Finally we write down the subnet address of each subnet continuing where the previous subnet left off:
60 - 6 - /26 - 192.168.1.0/26
20 - 5 - /27 - 192.168.1.64/27
20 - 5 - /27 - 192.168.1.96/27
2 - 2 - /30 - 192.168.1.128/30
2 - 2 - /30 - 192.168.1.132/30
2 - 2 - /30 - 192.168.1.136/30

Here's another practice question:
Subnet 172.16.16.0/20 to address 10 subnets (510,254,255,2,126,127,2,2,2,2) using the most efficient addressing possible.

First we arrange them descending:
510
255
254
127
126
2
2
2
2
2

Then we figure out how many host bits are required:
510 - 9
255 - 9
254 - 8
127 - 8
126 - 7
2 - 2
2 - 2
2 - 2
2 - 2
2 - 2

Then we figure out the mask:

510 - 9 - /23
255 - 9 - /23
254 - 8 - /24
127 - 8 - /24
126 - 7 - /25
2 - 2 - /30
2 - 2 - /30
2 - 2 - /30
2 - 2 - /30
2 - 2 - /30

Finally we write down the subnet address of each subnet continuing where the previous subnet left off:

510 - 9 - /23 - 172.16.16.0/23
255 - 9 - /23 - 172.16.18.0/23
254 - 8 - /24 - 172.16.20.0/24
127 - 8 - /24 - 172.16.21.0/24
126 - 7 - /25 - 172.16.22.0/25
2 - 2 - /30 - 172.16.22.128/30
2 - 2 - /30 - 172.16.22.132/30
2 - 2 - /30 - 172.16.22.136/30
2 - 2 - /30 - 172.16.22.140/30
2 - 2 - /30 - 172.16.22.144/30

In the Cisco certification, a typical question would be a drag and drop question which lets you drag subnets on the left to a diagram on the right.