The Supernet CIDR (Classless Inter-Domain Routing) chart serves as a reference tool that aids network administrators in understanding and implementing supernetting. It provides a systematic overview of supernetting and its associated CIDR notation, facilitating the calculation and identification of supernet addresses.

Below is our Supernetting/CIDR chart consisting of four columns: CIDR Block, Supernet Mask, Number of Class C Networks and the Number of Hosts:

The CIDR Block simply represents the number of bits used for the subnet mask. For example, /14 means 14 bits assigned to the subnet mask.

We should also take note at the CIDR Block entries /24, and /25 to /30. These blocks are highlighted in blue and yellow.

When we use a CIDR Block of 24 (24 bit subnet mask) we are not Supernetting because this is a default subnet mask for a Class C network. With CIDR Blocks /25 to /30 we are actually subnetting and not Supernetting.

CIDR blocks of /30 (4 IP addresses of which 2 are usable) are often assigned by ISPs to their customers and used to assigned their only two valid IP addresses to the equipment between the ISP and customer (usually routers or firewalls).

The Supernet Mask

Basically, this is your subnet mask. When you configure the devices that will be attached to the specified network, this is the value you will enter as a subnet mask. It's also the decimal value the CIDR Block specifies. For example, a CIDR block of /24 means a 24bit subnet mask, which translates to 255.255.255.0.

To understand the coversion of binary bits to decimal numbers, read our article How Supernets Work.

Number of Class C Networks

This number shows us how many Class C Networks are combined by using a specific Supernet mask or, if you like, CIDR Block. For example, the /24 CIDR Block, 255.255.255.0 Supernet mask is 1 Class C Network, whereas a /20 CIDR Block, 255.255.240.0 Supernet mask is 16 Class C networks.

Number Of Hosts

This value represents the number of hosts per Supernet. For example, when we use a /20 CIDR Block, which means a Subnet (or Supernet) mask of 255.255.240.0, we can have up to 4096 hosts.

Keep in mnd that the value of 4096 does not represent the number of valid (or usable) IP addresses. To calculate the number of valid IP addresses, you'll need to subtract two IPs (network & broadcast IP): 4096-2 =4094.

Summary

That completes our discussion on the Supernetting/CIDR chart. You will see that Supernetting and Subnetting have quite a few things in common, and this is simply because they work on the same principle.

Again, if you have the whole topic, or certain sections hard to understand, you should give yourself a small break, and then come back for another round :)

Article Key Topics:

• Guideline & Rules to Supernetting/CIDR.
• Supernetting Example and Analysis.
• Summary.

It is highly advisable to read through our article  What is Supernetting (Route Summarization) & How Supernets Work    if you haven't already done so.

Supernetting requires solid understanding of IP, subnetting and binary notation. Read through these sections if in doubt.

Guidelines & Rules to Supernetting / CIDR

Before we get in to deep waters, we must talk about the main rule that applies to creating Supernets. For our example, this rule dictates that, in order to create Supernets from Class C IP addresses, the network address must be consecutive and the third octet of the first IP Address must be divisible by two.

If we had 8 networks we wanted to combine, then the third octet of the first IP address would need to be divisible by eight and not two.

There is one more rule you should know and this rule has to do with the routers of the network, which will need to work with the new changes. This rule dictates that all routers on the network must be running static routing or using a classless routing protocol such as RIPv2 or OSPF. Classless routing protocols include the subnet mask information and can also pass supernetting information. Routing protocols such as RIPv1 don't include subnet mask information and would certainly create major routing issues between the networks.

When attempting to calculate a subnet or supernet, it is always advisable to reference a Supernet/CIDR chart to assist with the planning and correct calculation of networks, subnets and hosts.

Supernetting Example and Analysis

Here is an example involving two companies that want to use Supernetting to solve their network requirements. We are going to determine which company mets the criteria for a Supernet (we are assuming the routers are setup in a way that will support supernetting):

The network for company 1 passes the test requirements and is therefore an eligible candidate for Supernetting.

Creating our first Supernet

Let's now take Company No.1's network, see how the Supernet will be created and determine various important parameters like the new network's broadcast address, the identification of the new supernets etc.

To begin, we must take our two networks and look at them in binary format, this is the only way to "see" exactly what we're doing when supernetting, and take a look at the Network and Host ID portions:

If you have problems understanding why we have no Subnet ID, please read up on the IP  and Subnetting sections on this site where everything is explained as simply as possible using cool 3D diagrams.

Now we need to create the Supernet. This means that we are going to take one bit from the Network ID of these networks and give it to the Host ID portion. This 1bit is our Supernet ID. So our subnet mask will now be reduced from 24 bits to 23 bits. You might get confused or ask why we call this extra Bit we are giving to the Host ID a Supernet ID?

The answer is simple, the one bit that we are taking from the Network ID is given to the Host ID but, in order for us to clearly "see" where the supernet is created, we colour it green and give it the "Supernet ID" label:

So there you have it, a new supernet created!

Now we can point out something new; that we intentionally left out, in order not to confuse matters!

We have one Supernet made from two networks (203.31.218.0 and 203.31.219.0). In order to identify these two networks we name the first one (203.31.218.0) Supernet 0 and the second one (203.31.219.0) Supernet 1. This is to simply distinguish between the two networks.

It actually makes more sense if you look at the values the Supernet ID field takes:

It's very important to understand that Supernet 0 and 1 are part of the same network! This means that there is only one network address, one network broadcast address.

Let's now have a look at some more important information regarding the new network:

The above table shows pretty much all the information someone would need about the new network.

It is also worth nothing that following boardcast IP addresses (based on the original network): 203.31.218.255 and 203.31.219.0 are now valid IP addresses that can be assigned to hosts.

Also, every host that will be part of this Supernet will need to be configured with the new subnet mask, 255.255.254.0 as noted in the table above. Any host that isn't reconfigured will experience network communication issues. 

Summary

In this article we talked about Supernetting guidelines and rules and provided tips on how to supernet a network. We covered extensively a supernet example and calculated all necessary parameters.

Article Key Topics:

• The Purpose of Supernetting - Supernet Example.
• Supernetting Advantages.
• How Supernetting Works - Comparing Supernetting & Subnetting.
• Summary.

Supernetting requires solid understanding of IP, subnetting and binary notation. Read through these sections if in doubt.

The Purpose of Supernetting - Supernet Example

Supernetting exists to address the limitations of the traditional classful addressing scheme. In classful addressing, IP addresses are divided into fixed classes (A, B, C, etc.) with predefined subnet sizes. However, this approach often leads to IP address wastage and inefficient use of available address space. Supernetting, on the other hand, allows for the creation of custom-sized subnets, breaking free from the constraints of classful addressing.

To illustrate supernetting, let's consider an example. Suppose an organization has been assigned the following IP address blocks: 192.168.0.0/24, 192.168.1.0/24, and 192.168.2.0/24. Instead of advertising these three separate /24 networks, supernetting allows us to aggregate them into a single larger network. By applying supernetting, we can represent these networks as a supernet with the address block 192.168.0.0/22. This means that the organization can advertise the supernet address to the external network, simplifying routing tables and reducing overhead.

Let's take a look at second, bigger scale example. The diagram below represents an ISP with multiple customer networks reachable via the internet. Without Supernetting, or Route Summarization, the ISP Router 1 would require one route entry for each client's network:

Taking into consideration ISPs usually have hundreds and thousands of customers, we would need one entry for each of these networks in Router1's routing table. This creates an enourmous requirement in memory and processing power to maintain and update.

Routers 1 & 2 exchange information with each other and update their routing tables. Router 2 connects directly to 10 networks and needs to let Router 1 know about each one of them. Router 1 in turn will also advertise these networks to the internet backbone router so the internet is aware on how to reach these remote networks.

With Supernetting hundreds or thousands of route entries can be theoretically replaced with a single route entry that includes all these remote client networks.

The diagram below visually illustrates this concept. Notice how all client networks have been combined into one larger network:

Even though Router 1 and the internet backbone router summarize all client networks into one (larger) network, Router 2 is still aware of each individual client network.

In conclusion, supernetting is a powerful technique that enables the efficient utilization of IP address space and optimizes routing in networks. By aggregating smaller subnets into larger supernet addresses, organizations can streamline their routing infrastructure and improve overall network performance. Supernetting offers a flexible and scalable solution to address the growing demands of modern networks while maximizing IP address allocation.

Supernetting Advantages

Supernetting has gained significant popularity in the world of networking due to several compelling reasons. Let's delve into why supernetting is widely embraced and valued in the networking community:

• Efficient IP Address Utilization: Supernetting allows for the aggregation of multiple smaller subnets into larger network blocks. This results in more efficient utilization of IP address space. By reducing the number of individual network entries in routing tables, supernetting helps conserve IP addresses, particularly in environments where address shortages are a concern.
• Simplified Routing Infrastructure: Supernetting simplifies routing tables by minimizing the number of route entries. Instead of advertising multiple smaller subnets, a supernet can represent those subnets as a single entry. This simplification improves the efficiency of routing protocols and reduces the complexity associated with managing and updating routing tables.
• Improved Routing Convergence: With supernetting, routing convergence becomes faster and more efficient. By summarizing multiple routes into a single supernet entry, routing protocols can converge more quickly as they only need to consider and update a smaller number of summarized routes. This contributes to reduced network convergence times, which is crucial for maintaining optimal network performance.
• Enhanced Network Security: Supernetting helps improve network security by reducing the exposure of specific network details to external entities. By advertising a supernet address instead of individual subnets, network administrators can hide the internal network structure and make it more challenging for potential attackers to gain insights into the network's internal topology.
• Scalability and Manageability: Supernetting enhances network scalability by reducing the size and complexity of routing tables. As networks grow in size and complexity, the management of routing information becomes more challenging. Supernetting provides a scalable solution by consolidating multiple subnets into summarized routes, easing the burden on routers and network administrators.

We can see that Supernetting provides a wide-ranging advantages, and is therefore no seceret why it has become a fundamental technique for optimizing network performance and addressing the challenges posed by the growth and complexity of modern networks.

How Supernetting Works

As mentioned at the beginning of our article, Supernets are the opposite of Subnets in that they combine multiple Class C networks into blocks rather than dividing them into segments.

With the Subnetting process, we borrow bits from the Host ID portion, which increases the number of bits used for the Network ID portion. With Supernetting we perform the opposite, meaning we take the bits from the Network ID portion and give them to the Host ID portion, as illustrated in the picture below:

Our article Supernetting In-Depth Analysis shows how to calculate supernets by converting IP addresses from decimal to binary format and applying subnetmasks.

Summary

This article explained the concept of Supernetting and provided a few examples to show how its used by organizations to combine multiple networks into one. We also covered the main advantages supernetting provides: efficient IP address utilization, simplified routing infrastructure, improved routing convergence, enhanced network security, scalability and manageability and finally saw how supernetting works.