---

---

I am an IT Director in the construction industry.
I enjoy management, and I enjoy high level infrastructure IT. The way I see myself being able to bridge these two loves is via the CCIE certification. This journal is for my own personal satisfaction. I do a lot more than just CCIE studies. Some of the technology I work with every day includes: Microsoft (no duh, but mostly server technology), Citrix, NetApp, VMWare, HP, Linux, and so on. Because of this, I may randomly post interesting things going on around me such as Enterprise Vault Exchange Deployments, WAN Optimization Projects, BES best practices, or even patch deployment, DataONTAP hacks, etc.

---

John's blog can be read at http://www.johnpatricklockie.com/

VLSM is simple if you think of it this way.

I have this subnet

10.0.0.0 - 10.0.3.255 (255.255.252.0)

I want to tell an application to scan a range of IPs, but this application doesn’t like to get ranges. It only works with netmasks. So if I tell it 10.0.0.0 /22 then it will scan all 1,024 host address (or 1,022 usable hosts).

VLSM is a way of subneting an already subneted address. Suppose I want my machine to scan for IP’s 10.0.3.0 – 10.0.3.100 because I know the host I am looking for should be in that range. Well, instead of scannng /22 I can feed the application this IP subnet:

10.0.3.0 /25 (255.255.255.128)

I just subneted my already subnet address 10.0.0.0 /22 in to a smaller network of 126 usable host address. Now my application will scan for 10.0.3.1 – 10.0.3.126. That’s much better than scanning the entire /22 subnet! This is kind of how route summarization works, only the other direction (taking lots of smaller routes to smaller networks and summarizing them as larger blocks).

Well, we could just as easily segment a network at layer 3 by using VLSM in this way. There is no smoke and mirror and no mystic guide required…

This is how I understand VLSM, and I think if you can come to grasp subnets then VLSM is simply an extension of that. You must look at your IPv4 addresses as blocks of usable IP’s in which you can build layered networks to manage your broadcast domains even better…

Another real world example of this, is if you had to give IP’s to 75 clients. Well, then you are forced to use the /25 subnet which contains 126 host IP’s. That’s a waste of 51 IP’s! So why not use VLSM to pick up these IP’s and use them in other networks which do not have as much an abundance of usable host IP’s?

Well, we will be using these IP:

10.0.0.1 - 10.0.0.75 /25

We will not be using these IP:

10.0.0.76 - 10.0.0.126 /25

Suppose our new network will require 28 hosts. We will want to subnet somewhere in the range of .76 and .126, 28 IP’s. We know from our subnet rules that to get 28 hosts requires we go with a 32 host network which is 255.255.255.224 (/27). We need to determine the start and end ranges, which is pretty easy to do:

10.0.0.1 - 10.0.0.30 ‹-- contains IP's in our 10.0.0.1 - 10.0.0.75 network
10.0.0.33 - 10.0.0.62 ‹-- contains IP's in our 10.0.0.1 - 10.0.0.75 network
10.0.0.65 - 10.0.0.94 ‹-- contains IP's in our 10.0.0.1 - 10.0.0.75 network
10.0.0.97 - 10.0.0.126 ‹-- first free block outside our already used IP's!

Therefore, we have the following VLSM scheme of contained IP’s within the /25 subnet:

Host Network A

Network: 10.0.0.0 /25
Broadcast: 10.0.0.127 /25
Host Range: 10.0.0.1 - 10.0.0.126
Actual Used IP's: 10.0.0.1 - 10.0.0.75

Host Network B (subnet the remaining host IP’s in the /25 network to get 28 hosts an a new network)

Network: 10.0.0.96 /27
Broadcast: 10.0.0.127 /27
Host Range: 10.0.0.97 - 10.0.0.126
Actual Used IP's: 10.0.0.97 - 10.0.0.125

Now that is IP efficiency!

Return to the top of 'VLSM Concepts…'.
Send Feedback