VLSM is doing my head in!!!!!!

Vex10

Hey guys,

Does VLSM show up in the exam, I cant seem to understand it.

Any help would be greatly appreciated.

Vex10

dynamik

Multiple study resources that I'm using explicitly says it does not, but I think I've come across a VLSM question or two in Transcender.

Be sure to convert to binary, at least while you're getting started. Going about it that way makes it pretty straight forward.

Vex10

Cool thanks for the heads up.

mrx9000

I agree, I always write it down - 11111111.11111111.11111111.11111111

Claymoore

By VLSM do you mean the /?? notation (commonly refered to as CIDR notation) or do you mean subnets other than the traditional classful boundaries such as 255.255.255.0? Or do you really mean using different classless subnets within the same classful boundary, such as a mixture of /28 and /27 subnets in a 192.168.1.0 classful network? I don't specifically remember the test using CIDR notation, but you can certainly expect a 10.0.0.0 network to be subnetted in all sorts of different ways. You need to understand subnetting, but try not to get too wrapped up in the network IDs and masks listed in the questions if you only really need to know where to install a DHCP relay agent.

Here is a VLSM example from a Cisco Technote:

VLSM Example
Given the same network and requirements as in Sample Exercise 2 develop a subnetting scheme with the use of VLSM, given:

netA: must support 14 hosts
netB: must support 28 hosts
netC: must support 2 hosts
netD: must support 7 hosts
netE: must support 28 host

Determine what mask allows the required number of hosts.

netA: requires a /28 (255.255.255.240) mask to support 14 hosts
netB: requires a /27 (255.255.255.224) mask to support 28 hosts
netC: requires a /30 (255.255.255.252) mask to support 2 hosts
netD*: requires a /28 (255.255.255.240) mask to support 7 hosts
netE: requires a /27 (255.255.255.224) mask to support 28 hosts

* a /29 (255.255.255.24 would only allow 6 usable host addresses therefore netD requires a /28 mask.

The easiest way to assign the subnets is to assign the largest first. For example, you can assign in this manner:

netB: 204.15.5.0/27 host address range 1 to 30
netE: 204.15.5.32/27 host address range 33 to 62
netA: 204.15.5.64/28 host address range 65 to 78
netD: 204.15.5.80/28 host address range 81 to 94
netC: 204.15.5.96/30 host address range 97 to 98

Also, learntosubnet.com is a useful site for, well, learning to subnet.

Vex10

Thank you Claymoore you have cleared that up very good.

The cisco technotes is good to, thank you again.

Vex10

mattd75

How do you have a CCNA and not understand VLSM? I assumed that would be a huge part of passing any Cisco exam?

Vex10

It was but that was ages ago and I have not used it in along time, with me my head is like a bucket with holes. If I dont use it goes. Even the it took me ages to understand it through reading it over and over.

Dracula28

I had a VLSM related question on the exam, so it can show up on the exam, even if the training kit says that its highly unlikely.