VLSM addressing and alternate CIDR

[John Cheetley]

Hi again team,

Just done a practice exam for ICND1.

Got some questions wrong.

1st is:

Which of the following options are examples of VLSM?
A. Taking the 192.168.1.0/27 and subnetting it to multiple /30s
B. Taking 192.168.1.0/24 and subnetting it to multiple /29s
C. Taking 172.16.1.0/24 and subnetting it to multiple /26s
D. Taking 10.1.24.128/26 and subnetting it to multiple /28s
E. Taking 172.16.0.0/16 and subnetting it to multiple /24s

I wasn't aware that the CIDR number could be changed ?The question clearly shows 2 separate CIDR numbers

Thanks in advance again :) 

[Julio E. Moisa]

Hi John,

I am not really sure if it is a question of a exam but the key to understand VLSM is that you need to waste the least amount of IP addresses during the subnetting in order to complete with the requests.

The idea with VLSM is make subnetting to complete targets, so if you are subnetting to complete a target, you can take a leftover network to complete other target. 

Take in consideration a specific request says: I want a subnet with 28 hosts, you can use a /27 instead a /26: if the CIDR is /27 (30 valid IP addresses, 32-2) it will have least valid IP addresses than a /26 (62 valid IPaddress 64-2), in few words you are not wasting around 30+ IP addresses.

Please see the attached file.

Hope it is useful

:-)

[John Cheetley]

Yes it was practice exam question . I thought it was a different type of vlsm question . Thanks for the help in clarifying this 

[John Cheetley]

Thanks for the quick response Julio,

Yes it was a question from practice exam. I understand the purpose of VLSM and minimum ip address wastage. Was just trying to understand changing from /24 to the /30.