Variable Length Subnet Masking (VLSM)

Maltenrazer

is it worth trying to learn about VLSMs? do questions like this even com up?

meadIT

Yes, they do, but I don't think you would have more than 1 or 2, so if you know the rest of your stuff pretty well, you could theoretically still pass without knowing VLSM.

dynamik

If you have subnetting down, you really shouldn't have a problem with VLSMs. Just do some practice and perfect the technique. Your learning goals should be to master the material, not simply pass the exam.

Maltenrazer

i got my exam booked for 5th... wish me luck and some advice on specific things i should go over would b much appreciated.

any good resources on VLSMs on the net? i found this http://www.tcpipguide.com/free/t_IPVariableLengthSubnetMaskingVLSM.htm the more the merrier i say. i will go over it tomorrow.

meadIT

www.learntosubnet.com

Also check out royal's resources thread

royal

Like dynamik said, VLSM is easy once you know how to subnet.

So let's say you have a bunch of subnets. Some have many computers but some subnets only have 20 or so computers. So let's take a look

For example, default Class C is /24. So let's say you're subnetting and borrowing 2 bits. /26
1111111.11111111.11111111.11000000

With this we have 6 client bits. That's 2^6-2. You can have 64 clients in each subnet. But, some subnets only have 20 computers and you know for sure that you won't be adding more machines. Why waste 44 client IPs? Instead, we can use VLSM which is kind of like subnetting your subnets.

Since we're working with /26, we own that entire subnet. Because of that, anything beyond the /26 mark we can do whatever we want with it. Ok, so let's make a /27, /28, and a /29.

We now segmented our network even more. Instead of just having 2 bits for a subnet, we just increased the amount of subnets even more. Because a /27 will have more client IPs, we can provide that range (that specific subnet) to the group of computers that need that many IPs. So now you are being more efficient and not wasting 44 client IPs.

Make sense?