Understanding VLSM

[mahmoodmkl]

Hi Guys,

Well one more basic question as i m brushing my basics.

I am a bit confused about how vlsm works.Below is the example which through which i want to understand it.

Lets say i have a network of 192.168.1.0/24 which i need  to divide as below.

60 hosts

20 hosts

20 hosts

and three ranges  for wan subnets.

I understand that if i take 2 bits from the host portion i will have 192.168.1.0 - 63 with a mask of 26 which is enough for the 1 subnet.

further going on if i take 3 bits from the host portion from here my confusion starts.

the first subnet ranges were 192.168.1.0,192.168.1.64,192.168.1.128 etc.

now for the second subnet range the subnets will start from 192.168.1.0,192.168.1.32 etc.But as per documentation i cannot use this ranges.

I want to know why i cannot use these ranges as they have different subnet mask.

Please explain.

Thanks

[cadet alain]

Hi,

when you do vlsm once you've found the subnets/mask for the highest number of hosts and used the first subnet then you have to subnet again the second free subnet:

in your case 192.168.1.0/24 you subnet for 60 hosts so it gives a /26 and the subnets are

192.168.1.0-192.168.1.64-192.168.1.128-192.168.1.192

As you've used 192.168.1.0/26 for the first subnet with 60 hosts you can only subnet next free subnet which is 192.168.1.64/26 for 20 hosts so it will give /27 which gives the following subnets:

192.168.1.64/27-192.168.1.96/27 and so on so you'll use the first 2  for the 2 20 hosts subnets.

Regards.

Alain.