Well, let’s first get down to the basics and define a few keywords here. A network is a group of connected devices that allows for the sharing of resources and information. Each device on the network is identified by what we call an Internet protocol (IP) address. All devices are connected to a single network, either physically or wirelessly connecting to a single device called a switch or wireless access point. The definition of all common devices connecting via a single medium is called a local area network (LAN).
With all devices connected to each other via cable or wireless, the question beckons, how do these devices communicate with each other?
Well, these connected devices send data between each other by addressing each other by their IP address. The sender’s and recipient’s IP address are both found in the data stream that flows between the sender and receiver. This information therefore allows the receiver to respond to the sender.
So by this oversimplified description of networking, we can take from this that the IP address is an important piece of the puzzle to allow devices to communicate on a single network.
Now, with that fun fact in your pocket, how does this bring us back on track to the topic at hand? Well, let’s hit you with another definition, shall we?
A vlan (pronounced V LAN), like a LAN is a “virtual” local area network which means that you no longer need a physical device acting as a connecting medium between all other networked devices per LAN. All that is different in this scenario, is that each virtual LAN is represented by what we call a vlan tag… a number value between 1 and 4096.
Now, there are more in-depth concepts to know about networking, like “IP subnetting” which we will not go through in this post although it is crucial to actually realizing vlan segmentation. Check out the CCENT courses on cbtnuggets by Jeremy Cioara for a detailed look at how to subnet like a boss. Here is a link to a few examples covering Jeremy’s method of subnetting.
The basics thou on subnetting is that you can calve out slices of a network subnet into smaller pieces to then make up multiple smaller subnets. This way you can save IP addresses and still keep devices protected from talking to each other by placing them into their own subnet or vlan. Each subnet makes up a vlan. Each vlan plays within its own IP address space. Devices in a single vlan can send broadcast messages to every device only within the boundaries of its own vlan.
Therefore an additional benefit to vlan segmentation is the reduction of broadcast packets on the Network. This is great for improving network performance.
So let’s get started with the shopping list shall we?
What makes a good planning session for vlan segmentation… here is your check list below:
> Define your starting subnet size. What is the network subnet that you will be breaking up into smaller networks?
> How many different networks do you need at the one site?
> How many devices do you need to cater for in each smaller network?
The Network block sizes starts from 2 and works its way up to 128 in an exponential to the power of two. E.g. 2, 4, 8, 16, 32, 64 & 128
In each network block size, you will need to reserve the first IP address for the Network identifier and the last IP address as the broadcast IP address therefore we can take the above increment examples and subtract 2 from it to get a true available count of IP addresses to use in your smaller subnets.
Illustration 1: Block Size Template
In the illustration above you can see that in each subnet block the next number from zero increases by the amount denoted by the block size, so if the subnet block is 4, you see increments of 4 until you reach the end denoted by the number 256.
> Now that you have defined how many networks you need and the size of your smaller networks, it’s time to line them up in descending counts. Let us explain this with a small scenario:
Scenario 1: You have a Network subnet of 192.168.0.0/24. This equates to 254 available IP Addresses to allocate to networked devices. You have to break this large subnet up into 5 smaller networks of different sizes to allocate for the different types of networked devices in your office. You have to cater for at least 80 client pc’s, about 20 printers and 9 wireless access points that offer 2 different Wi-Fi networks that would each need to cater for at least 25 clients.
So to summarize, I have 5 networks that I need to cater for in descending order from 80 desktop clients, 2 x 25 wireless clients, 20 x printers and lastly 9 wireless access points. When applying these numbers to the Block Size template, you must choose a block size that is larger than the amount of clients needing IP addresses for that network. Therefore I would need at least 1 x block 128, 3 x block 32 and 1 x block 16.
Always start your largest network from dot zero. Therefore the network range for the PC’s would be from 192.168.0.0 – 192.168.0.127. Remembering that the first IP and the last IP in a range should be reserved as network identifier and the broadcast IP, you then land up with the following definition for the first IP block:
Block Size 128 Subnet: 192.168.0.0/25 (Slash Notation Representation)
192.168.0.0: Network Identifier
192.168.0.1 – 126: Available IP’s in Block Size 128 subnet
192.168.0.127: Broadcast IP
The easier way to write down a network subnet is by slash notation. The cheat sheet for slash notation for the different block sizes is below:
From the range of available IP’s you would need to select which IP in the range would be best suited as your default gateway for that Network block. A word of advice, would be to always either choose the first available IP address in the range or the last available IP in the range.
After sorting your required Network blocks in descending order and defining your Network identities, gateways and broadcast IP addresses, you would have, at this point defined your blueprint for vlan segmenting your Network.
Scenario 1: Vlan Segmentation Example Blueprint
All that’s left to do now is to configure your network as per your blueprint and switch on your network!