Cool Subnetting Tricks with VLSM

A few months back, I showed you how to organize your network into smaller subnets. My post covered the details of the concept of subnetting. So if you missed that article, I would suggest taking a look at it to make sure you understand VLSM and this article in its entirety. For now, I will assume that you are already familiar with subnetting and know how to divide a network into smaller subnets.
In today’s article, we’ll subnet an already subnetted network into multiple subnets with variable subnet masks and then allocate them within our sample network.
Variable Length Subnet Mask (VLSM) is a key technology on large scalable networks. Mastering the concept of VLSM is not an easy task, but it’s well worth it. The importance of VLSM and its beneficial contribution to networking design is unquestionable. At the end of this article you will be able to understand the benefits of VLSM and describe the process of calculating VLSMs. I will use a real world example to help you understand the whole process and its beneficial effects.

Benefits of VLSM

VLSM provides the ability to subnet an already subnetted network address. The benefits that arise from this behavior include:
Efficient use of IP addresses: IP addresses are allocated according to the host space requirement of each subnet.
IP addresses are not wasted; for example, a Class C network of 192.168.10.0 and a mask of 255.255.255.224 (/27) allows you to have eight subnets, each with 32 IP addresses (30 of which could be assigned to devices). What if we had a few WAN links in our network (WAN links need only one IP address on each side, hence a total of two IP addresses per WAN link are needed).
Without VLSM that would be impossible. With VLSM we can subnet one of the subnets, 192.168.10.32, into smaller subnets with a mask of 255.255.255.252 (/30). This way we end up with eight subnets with only two available hosts each that we could use on the WAN links.
The /30 subnets created are: 192.168.10.32/30, 192.168.10.36/30, 192.168.10.40/30, 192.168.10.44/30, 192.168.10.48/30, 192.168.10.52/30, 192.168.10.56/30 192.168.10.60/30.
Support for better route summarization: VLSM supports hierarchical addressing design therefore, it can effectively support route aggregation, also called route summarization.
The latter can successfully reduce the number of routes in a routing table by representing a range of network subnets in a single summary address. For example subnets 192.168.10.0/24, 192.168.11.0/24 and 192.168.12.0/24 could all be summarized into 192.168.8.0/21.

Address Waste Without VLSM

The following diagram shows a sample internetwork which uses a network C address 192.168.10.0 (/24) subnetted into 8 equal size subnets (32 available IP addresses each) to be allocated to the various portions of the network.
This specific network consists of 3 WAN links that are allocated a subnet address range each from the pool of available subnets. Obviously 30 IP address are wasted (28 host addresses) since they are never going to be used on the WAN links.

Implementing VLSM

In order to be able to implement VLSMs in a quick and efficient way, you need to understand and memorize the IP address blocks and available hosts for various subnet masks.
Create a small table with all of this information and use it to create your VLSM network. The following table shows the block sizes used for subnetting a Class C subnet.

Having this table in front of you is very helpful. For example, if you have a subnet with 28 hosts then you can easily see from the table that you will need a block size of 32. For a subnet of 40 hosts you will need a block size of 64.

Example: Create a VLSM Network

Let us use the sample network provided above to implement VLSM. According to the number of hosts in each subnet, identify the addressing blocks required. You should end up with the following VLSM table for this Class C network 192.168.10.0/24.

Take a deep breath … we’re almost done. We have identified the necessary block sizes for our sample network.
The final step is to allocate the actual subnets to our design and construct our VLSM network. We will take into account that subnet-zero can be used in our network design, therefore the following solution will really allow us to save unnecessary addressing waste:

With VLSM we have occupied 140 addresses. Nearly half of the address space of the Class C network is saved. The address space that remains unused is available for any future expansion.
Isn’t that amazing? We have reserved a great amount of addresses for future use. Our sample network diagram is finalized as shown on the following diagram:

Final Thoughts

Variable Length Subnet Mask is an extremely important chapter in Network Design. Honestly, if you want to design and implement scalable and efficient networks, you should definitely learn how to design and implement VLSM.
It’s not that difficult once you understand the process of block sizes and the way to allocate them within your design. Don’t forget that VLSM relates directly to the subnetting process, therefore mastering the subnetting process is a prerequisite for effectively implementing VLSM. And feel free to go through my subnetting articles a couple of times to get a hang of the whole process.

By Stelios Antoniou
http://www.trainsignal.com/blog/cisco-ccna-vlsm

How to Organize Your Network Into Smaller Subnets

In my last article, IP Addressing and Routing Part 1: The Invasion of IP Addresses, I presented the architecture of the IP addressing scheme. We went over the IP Network Classes and how to distinguish between them.
If you’re new to this field, I would suggest adding both Part 1 and Part 2: IP Routing Process to your reading list, since it provides some additional information that can be useful in getting a firm grasp of the subnetting concept.
In today’s article we are going to learn about the concept of subnetting and how we can use it to divide our classful network into smaller networks that can operate in separate working zones. We’ll also take a look at how we can conserve address space and save resources on process management with the use of subnetting.
I’ll use a few examples to clearly present the steps of subnetting and help you master this topic. And although at first this may seem difficult, don’t give up! All it takes is some time and practice!

What Is Subnetting?

Subnetting is the process of stealing bits from the HOST part of an IP address in order to divide the larger network into smaller sub-networks called subnets. After subnetting, we end up with NETWORK SUBNET HOST fields. We always reserve an IP address to identify the subnet and another one to identify the broadcast address within the subnet. In the following sections you will find out how all this is possible.

Why Use Subnetting?

Conservation of IP addresses: Imagine having a network of 20 hosts. Using a Class C network will waste a lot of IP addresses (254-20=234). Breaking up large networks into smaller parts would be more efficient and would conserve a great amount of addresses.
Reduced network traffic: The smaller networks created the smaller broadcast domains are formed hence less broadcast traffic on network boundaries.
Simplification: Breaking large networks into smaller ones could simplify fault troubleshooting by isolating network problems down to their specific existence.

The Subnetting Concept

You will be surprised how easy the concept of Subnetting really is. Imagine a network with a total of 256 addresses (a Class C network). One of these addresses is used to identify the network address and another one is used to identify the broadcast address on the network. Therefore, we are left with 254 addresses available for addressing hosts.
If we take all these addresses and divide them equally into 8 different subnets we still keep the total number of original addresses, but we have now split them into 8 subnets with 32 addresses in each. Each new subnet needs to dedicate 2 addresses for the subnet and broadcast address within the subnet.
The result is that we eventually come up with 8 subnets, each one possessing 30 addresses available for hosts. You can see that the total amount of addressable hosts is reduced (240 instead of 254) but better management of addressing space is gained. I’ll now use a couple of examples to help explain the process of subnetting as clearly as possible.

Subnetting a Class C Address Using the Binary Method

We will use a Class C address which takes 5 bits from the Host field for subnetting and leaves 3 bits for defining hosts as shown in figure 1 below. Having 5 bits available for defining subnets means that we can have up to 32 (2^5) different subnets.
It should be noted that in the past using subnet zero (00000—) and all-ones subnet (11111—) was not allowed. This is not true nowadays. Since Cisco IOS Software Release 12.0 the entire address space including all possible subnets is explicitly allowed.

Let’s use IP address 192.168.10.44 with subnet mask 255.255.255.248 or /29.

STEP 1: Convert to Binary

STEP 2: Calculate the Subnet Address

To calculate the Subnets IP Address you need to perform a bit-wise AND operation (1+1=1, 1+0 or 0+1 =0, 0+0=0) on the host IP address and subnet mask. The result is the subnet address in which the host is situated.

STEP 3: Find Host Range

We know already that for subnetting this Class C address we have borrowed 5 bits from the Host field. These 5 bits are used to identify the subnets. The remaining 3 bits are used for defining hosts within a particular subnet.
The Subnet address is identified by all 0 bits in the Host part of the address. The first host within the subnet is identified by all 0s and a 1. The last host is identified by all 1s and a 0. The broadcast address is the all 1s. Now, we move to the next subnet and the process is repeated the same way. The following diagram clearly illustrates this process:

STEP 4: Calculate the Total Number of Subnets and Hosts Per Subnet

Knowing the number of Subnet and Host bits we can now calculate the total number of possible subnets and the total number of hosts per subnet. We assume in our calculations that all-zeros and all-ones subnets can be used. The following diagram illustrated the calculation steps.

Subnetting a Class C Address Using the Fast Way

Now let’s see how we can subnet the same Class C address using a faster method. Let’s again use the IP address 192.168.10.44 with subnet mask 255.255.255.248 (/29). The steps to perform this task are the following:
1. Total number of subnets: Using the subnet mask 255.255.255.248, number value 248 (11111000) indicates that 5 bits are used to identify the subnet. To find the total number of subnets available simply raise 2 to the power of 5 (2^5) and you will find that the result is 32 subnets.
Note that if subnet all-zeros is not used then we are left with 31 subnets and if also all-ones subnet is not used then we finally have 30 subnets.
2. Hosts per subnet: 3 bits are left to identify the host therefore the total number of hosts per subnet is 2 to the power of 3 minus 2 (1 address for subnet address and another one for the broadcast address)(2^3-2) which equals to 6 hosts per subnet.
3. Subnets, hosts and broadcast addresses per subnet: To find the valid subnets for this specific subnet mask you have to subtract 248 from the value 256 (256-248=8) which is the first available subnet address.
Actually the first available one is the subnet-zero which we explicitly note. Next subnet address is 8+8=16, next one is 16+8=24 and this goes on until we reach value 248. The following table provides all the calculated information.
Note that our IP address (192.168.10.44) lies in subnet 192.168.10.40.

Test Your Subnetting Knowledge and Practice, Practice, Practice!

Don’t get discouraged if you didn’t understand every little detail I went over in this article. Subnetting is not really that difficult, but it does require a bit of practice.
Start with testing your knowledge of subnets and make sure you feel confident about this before you move on to designing your own subnets. But remember, if you’re on the Cisco Networking track you will have to deal with subnetting sooner or later, so grab this opportunity and start testing yourself.
Go ahead and subnet the network address 192.168.10.0 address using the subnet mask 255.255.255.192 (/26). Find the valid subnets, host ranges and broadcast addresses per subnet. If you want to double-check your answer, feel free to leave me a comment and I will provide you with the correct solution.

By Stelios Antoniou
http://www.trainsignal.com/blog/simplify-routing-how-to-organize-your-network-into-smaller-subnets

Yorkshire Pudding

4 oz (100g) Plain Flour

1 medium sized egg

Pinch of salt

½ pint (280ml) of milk (or mixture of milk and water)

2oz (50g) lard/fat or 2 tablespoons of oil

Mix the flour and salt in a basin and make a hollow in middle. Drop the egg into the hollow and stir in with a wooden spoon. Add the milk (milk and water) gradually, stirring all of the time until the flour is worked in. Add rest of liquid and beat well. The end result should have a similar consistency to single cream.

Melt the fat in cooking tin until spitting hot.(top of the oven) Can be one large tin square, rectangular, round or small tins or a bun tin. When the fat is hot enough pour in the batter just half filling small tins, patty tins or bun tins. Cook at 230C or gas mark 8. Large tins for about 30 minutes, small tins or bun tins 15 – 20 minutes.

When cooked they should turn out puffy, golden and crispy on the outside and sunken in the middle. 

By Lorraine Waddington