This tutorial explains Supernetting, route summary and route aggregation in detail with examples.
Learn how Supernetting is done step by step as well as the basic and basic concepts of Supernetting such as what Supernetting is,
why Supernetting is done and what are the benefits of Supernetting.

What is Supernetting?

Supernetting is the process of summarizing a group of contiguous subnetworks into a single large network.
Supernetting is also known as route summary and route aggregation.

The following figure shows an example of Supernetting.

In the example above, 8 subnets are summarized into a single subnet.

Why is supernetting done?

Supernetting is mainly done to optimize routing tables. A routing table is the summary of all known networks.
Routers share routing tables to find the new route and locate the best route to the destination.

Without Supernetting, the router will share all routes from the routing tables as is. With Supernetting,
he will summarize them before sharing them. The route summary greatly reduces the size of the routing updates.

The following figure shows an example of a route summary.

example of route summary

Advantage of Supernetting

Supernetting offers the following advantages.

  • It reduces the size of routing updates.
  • It provides a better overview of the network.
  • It decreases the use of resources such as memory and processor.
  • It decreases the time required to rebuild the routing tables.

Supernetting components

Each route advertises a number of addresses, including the network ID, broadcast ID, and subnet mask.
We can use a term Block Size to collectively refer to all of these addresses.

In order to perform Supernetting, we need the network ID, the CIDR value, the broadcast ID, the subnet mask and the block size of each route.

  • The network ID and broadcast ID are used to check route alignment. Supernetting can only be done if the routes are sequential.
  • The block size is used to calculate the summary route from given routes.
  • The subnet mask and the CIDR value are the same thing in different notations. Both are used to find the ON network bits in the IP address. On the exam, the question can use any notation. When preparing for the Cisco exam, you should practice with both.

Since an advertising route is the combination of network ID and CIDR value, we just need to determine the
Broadcast ID, subnet mask and block size.

For the block size, use the following formulas: –

32 – CIDR Value = Number of host bits
Block size = 2Number of host bits

For example, if the CIDR value is 25, the block is 128.

The broadcast ID is the last address on the network. Once you know the size of the block, to calculate the delivery ID,
simply count the addresses from the network ID to the last address in the block.

For example, if the network ID is 192.168.1.0/25 and the block size is 128, then the broadcast ID will be 192.168.1.127/25.

When counting, the 0 is used as a number. For example, [0, 1 and 2] are 3 numbers.

The following table lists all of the CIDR values, as well as the subnet mask and block size.

Supernetting graphic

CIDR Subnet mask Block size
/ 8 255.0.0.0 16777216
/ 9 255.128.0.0 8388608
/ten 255.192.0.0 4194304
/ 11 255.224.0.0 2097152
/ 12 255.240.0.0 1048576
/ 13 255.248.0.0 524288
/ 14 255.252.0.0 262144
/ 15 255.254.0.0 131072
/ 16 255.255.0.0 65536
/ 17 255.255.128.0 32768
/ 18 255.255.192.0 16384
/ 19 255.255.224.0 8192
/ 20 255.255.240.0 4096
/ 21 255.255.248.0 2048
/ 22 255.255.252.0 1024
/ 23 255.255.254.0 512
/ 24 255.255.255.0 256
/ 25 255.255.255.128 128
/ 26 255.255.255.192 64
/ 27 255.255.255.224 32
/ 28 255.255.255.240 16
/ 29 255.255.255.248 8
/ 30 255.255.255.252 4

This tutorial is the last part of the article “Detailed explanation of IP subnets in a computer network with examples”. Other parts of this article follow.

This tutorial is the first part of the article. It explains IP addressing and network addressing such as IP address, subnet mask, IP address types and IP classes in detail.

Explanation of basic subnets in computer networks

This tutorial is the second part of the article. It explains what the subnet is and why it is needed in the computer network as well as the benefits of the subnet.

Subnet tutorial – Subnet explained with examples

This tutorial is the third part of the article. It explains in detail the concepts and terms of subnet such as network ID, broadcast ID, total number of hosts, valid hosts, power of 2, block size, and CIDR.

Subnet tips

This tutorial is the fourth part of the article. It explains the easiest and fastest way to perform subnets in Cisco exams and interviews.

VLSM subnet explained with examples

This tutorial is the fifth part of the article. It explains what the VLSM subnet is and how to do it step by step, as well as the differences between the FLSM subnet and the VLSM subnet.

Explanation of examples of VLSM subnets and calculation

This tutorial is the sixth part of the article. It explains the examples of VLSM subnets step by step in detail, including practical questions and answers about VLSM subnets.

Key points of Supernetting

Supernetting can only be done in the same address space. If the address space is completely different between two or more routes, they cannot be summarized in one route. For example, we cannot summarize route 192.168.1.0/25 with route 193.168.1.128/25.

A route can only be summarized in a route larger than it in block size. For example, we cannot summarize a block size route 64 in a block size route 32, but we can summarize two block size routes 32 in a single block size route 64.

The simplest way to calculate the summary route is to add the block size of all sequential routes and use the subnet that provides the block size which is equal to the result of the addition. For example, if we have two sequential routes of block size 16, we can summarize them into a single route of block size 32.

The synthesis can only be performed in the available bock sizes. For example, if we have 5 routes of block size 8, we cannot summarize them into a single road of block size 40 (8×5). 40 is not a valid block size. For valid block sizes, see the supernetting table above. In this case, the best choice is to summarize the first four routes, ie a single summarized route of block size 32 and to keep the fifth route as it is.

Like the block size, the network ID of the summarized route must match the network ID of the first sequential route.
To calculate the valid network in summary block size, just count in block size from 0.

For example, if the summarized block size is 32, the valid network IDs are 0, 32, 64, 96, 128, 160, 192 and 224. If the first sequential route of routes that we summarize does not start with any of those – network identifiers, they cannot be summarized in a single block size route 32, even if they meet the block size requirement.

For example, route 192.168.1.16/28 and route 192.168.1.32/28 cannot be summarized as a single route of block size 32, even if they are sequential and their collective block size (16 + 16) is equal to 32.

All sequential routes that start with one of these network IDs can be summarized with this block size. For example, route 192.168.1.0/28 and route 192.168.1.16/28 can be summarized as a single route 192.168.1.0/27 of block size 32

Never select a block size that does not cover all addresses, unless it is clearly mentioned that the remaining addresses will be used behind the router where the summary will be made.

For example, if we have two routes with a block size of 16 and 8, we cannot summarize them into one route of block size 32. If we do, the router will advertise a summarized route that says that router has a network path for 32 addresses when in reality it only has a network path for 24 (16 + 8) addresses.

Let’s take another example. If we have three routes with a block size of 16, instead of summarizing them all in one route of block size 64 (16 + 16 + 16 = 48), we need to summarize only the first two routes in one route block size 32 (16 + 16 = 32). In this case, the router will advertise two routes; a summary route of block size 32 and another original route. It is better to advertise two correct routes than to advertise a single incorrect route.

Examples of supernetting explained step by step

Above, we have taken the two examples of Supernetting. Let’s see how Supernetting was performed step by step.

Organize all routes in ascending order according to their value after slash (also known CIDR value).
If the CIDR value is the same on two or more routes, use their IP addresses for the command.

Supernetting example 1 After slash or CIDR value Example of supernetting 2 After slash or CIDR value
192.168.1.0/25 25 10.0.0.0/23 23
192.168.1.128/26 26 10.0.2.0/24 24
192.168.1.192/27 27 10.0.3.0/25 25
192.168.1.224/28 28 10.0.3.128/26 26
192.168.1.240/30 30 10.0.3.192/27 27
192.168.1.244/30 30 10.0.3.224/28 28
192.168.1.248/30 30 10.0.3.240/30 30
192.168.1.252/30 30 10.0.3.244/30 30
10.0.3.248/30 30
10.0.3.252/30 30

Enter the CIDR value, the subnet mask, the network ID, the broadcast ID and the block size for each route.

Supernetting example 1
Road CIDR value Subnet mask Network ID Broadcast ID Block size
192.168.1.0/25 25 255.255.255.128 192.168.1.0 192.168.1.127 128
192.168.1.128/26 26 255.255.255.192 192.168.1.128 192.168.1.191 64
192.168.1.192/27 27 255.255.255.224 192.168.1.192 192.168.1.223 32
192.168.1.224/28 28 255.255.255.240 192.168.1.224 192.168.1.239 16
192.168.1.240/30 30 255.255.255.252 192.168.1.240 192.168.1.248 4
192.168.1.244/30 30 255.255.255.252 192.168.1.244 192.168.1.247 4
192.168.1.248/30 30 255.255.255.252 192.168.1.248 192.168.1.251 4
192.168.1.252/30 30 255.255.255.252 192.168.1.252 192.168.1.255 4
Example of supernetting 2
Road CIDR value Subnet mask Network ID Broadcast ID Block size
10.0.0.0/23 23 255.255.254.0 10.0.0.0 10.0.1.255 512
10.0.2.0/24 24 255.255.255.0 10.0.2.0 10.0.2.255 256
10.0.3.0/25 25 255.255.255.128 10.0.3.0 10.0.3.127 128
10.0.3.128/26 26 255.255.255.192 10.0.3.128 10.0.3.191 64
10.0.3.192/27 27 255.255.255.224 10.0.3.192 10.0.3.223 32
10.0.3.224/28 28 255.255.255.240 10.0.3.224 10.0.3.239 16
10.0.3.240/30 30 255.255.255.252 10.0.3.240 10.0.3.243 4
10.0.3.244/30 30 255.255.255.252 10.0.3.244 10.0.3.247 4
10.0.3.248/30 30 255.255.255.252 10.0.3.248 10.0.3.251 4
10.0.3.252/30 30 255.255.255.252 10.0.3.252 10.0.3.255 4

Group the routes according to the sequence. If the network ID for a route starts from where the broadcast ID for the previous route ends,
it’s a sequential route. But if it does not start where the previous route ends, it is not a sequential route.

route summary table

Add the block size of all sequential routes.

In the first example, the sum of the block sizes is 256 and in the second example, it is 1024.

Check the nearest valid block size that provides an equal or lesser number of addresses. The size of blocks 256 and 1024 corresponds exactly to our needs. The subnet of / 24 and / 22 gives us the block size of 256 and 1024 respectively.

To write the summary route, use the network ID of the first route with the value CIDR or the subnet mask of the summary route.

In the first example, the network ID of the first route is 192.168.1.0 and the CIDR value of the summarized route is / 24. Thus, the route summarized for the first example will be 192.168.1.0/24.

Similarly in the second example, the network ID of the first route is 10.0.0.0 and the CIDR value of the summarized route is / 22. So the summary route for the second example will be 10.0.0.0/22.

Examples of complex supernetting

If IP addressing is properly planned, Supernetting is simple and straightforward as we have seen in the examples above. This only becomes difficult if unplanned IP addressing is used in the network.

If you are preparing for the Cisco exam, you should train with unplanned IP addressing. To test the caliber of candidates, Cisco generally puts complex and unplanned networks in questions related to Supernetting.

For an overview of the difficulty of the Supernetting questions in the Cisco exam, let’s see two examples of complex Supernetting. These examples are based on Supernetting questions asked in the CCNA Routing and Switching exam.

Example of supernetting 3

example of supernetting 3

Example of supernetting 4

example of supernetting 4

Step 1: – Organize all routes in ascending order.

Example of supernetting 3 Example of supernetting 4
Router A Router B Router A Router B
172.168.1.32/28 172.168.1.64/28 122.128.58.0/29 122.128.58.8/29
172.168.1.48/28 172.168.1.80/28 122.128.58.48/29 122.128.58.16/29
172.168.1.128/28 172.168.1.96/28 122.128.58.64/29 122.128.58.24/29
172.168.1.144/28 172.168.1.112/28 122.128.58.72/29 122.128.58.32/29
172.168.1.160/28 172.168.1.192/28 122.128.58.80/29 122.128.58.40/29
172.168.1.176/28 172.168.1.208/28 122.128.58.56/29

Step 2: – Enter the network ID, broadcast ID, CIDR value, subnet mask and block size for each route.

Example of supernetting 3 (router A)
Road CIDR Subnet mask Network ID Broadcast ID Block size
172.168.1.32 28 255.255.240.0 172.168.1.32 172.168.1.47 16
172.168.1.48 28 255.255.240.0 172.168.1.48 172.168.1.63 16
172.168.1.128 28 255.255.240.0 172.168.1.128 172.168.1.143 16
172.168.1.144 28 255.255.240.0 172.168.1.144 172.168.1.159 16
172.168.1.160 28 255.255.240.0 172.168.1.160 172.168.1.175 16
172.168.1.176 28 255.255.240.0 172.168.1.176 172.168.1.191 16
Example of supernetting 3 (router B)
Road CIDR Subnet mask Network ID Broadcast ID Block size
172.168.1.64 28 255.255.240.0 172.168.1.64 172.168.1.79 16
172.168.1.80 28 255.255.240.0 172.168.1.80 172.168.1.95 16
172.168.1.96 28 255.255.240.0 172.168.1.96 172.168.1.111 16
172.168.1.112 28 255.255.240.0 172.168.1.112 172.168.1.127 16
172.168.1.192 28 255.255.240.0 172.168.1.192 172.168.1.207 16
172.168.1.208 28 255.255.240.0 172.168.1.208 172.168.1.223 16
Example of supernetting 4 (router A)
Road CIDR Subnet mask Network ID Broadcast ID Block size
122.128.58.0 29 255.255.255.248 122.128.58.0 122.128.58.7 8
122.128.58.48 29 255.255.255.248 122.128.58.48 122.128.58.55 8
122.128.58.64 29 255.255.255.248 122.128.58.64 122.128.58.71 8
122.128.58.72 29 255.255.255.248 122.128.58.72 122.128.58.79 8
122.128.58.80 29 255.255.255.248 122.128.58.80 122.128.58.87 8
Example of supernetting 4 (router B)
Road CIDR Subnet mask Network ID Broadcast ID Block size
122.128.58.8 29 255.255.255.248 122.128.58.8 122.128.58.15 8
122.128.58.16 29 255.255.255.248 122.128.58.16 122.128.58.23 8
122.128.58.24 29 255.255.255.248 122.128.58.24 122.128.58.31 8
122.128.58.32 29 255.255.255.248 122.128.58.32 122.128.58.39 8
122.128.58.40 29 255.255.255.248 122.128.58.40 122.128.58.47 8
122.128.58.56 29 255.255.255.248 122.128.58.56 122.128.58.63 8

Step 3: – Based on the network ID and the broadcast ID, create the sequential route group.

grouping of similar routes

Step 4: – Summarize each group of sequential routes into one or more summarized routes.

  • Add the block size of all sequential routes in the group.
  • Look for the valid block of equal or smaller size.
  • From 0, count the valid block size and check whether the network ID of the first sequential route exists in the result or not.
  • If the network ID of the first sequential route exists in the result, use the valid block size to summarize the routes.
  • For synthesis, use the CIDR value which provides this block size.
  • If the network ID of the first sequential route does not exist in the result, use a smaller valid block size and count again. Repeat this step until the network ID of the first sequential route does not fall into the result.
Example of supernetting 3 (router A)

As we can see in the figure above, there are two groups of sequential routes in this router.

In the first group, there are two sequential routes; 32 and 48. The two routes have a block size of 16. The sum of the block sizes is 32 (16 + 16).
32 is a valid block size. The network ID of the first sequential route is 32, which is a valid network ID in block size 32 (0, 32, 64,…). The block size 32 is associated with the value CIDR / 27. Let us use this block size for the synthesis.

Summarize routes 172.168.1.32/28 and route 172.168.1.48/28 into a single route 172.168.1.32/27 of block size 32.

In the second group, there are 4 sequential routes 128, 144, 160 and 176 of block size 16. The sum of all block sizes is 64.
64 is a valid block size. The network ID of the first sequential route (128) is also in the block size range 64 (0, 64, 128, 192 ……).
So we can use block size 64 to summarize these routes. The CIDR value for block size 64 is / 26. Let’s use it to summarize these routes.

Summarize routes 172.168.1.128/28, 172.168.1.144/28, 172.168.1.160/28 and 172.168.1.176/28 in a single route 172.168.1.128/26 of block size 64.

Example of supernetting 3 (router B)

This router also has two groups of sequential routes. In the first group, there are 4 sequential routes 64, 80, 96 and 112
of block of size 16 and in the second group, there are 2 sequential channels 192 and 208 of block of size 16.

The sum of the block sizes is 64 (16 + 16 + 16 + 16) in the first group and 32 (16 + 16) in the second group. 64 and 32 are valid blocks
sizes and the network ID of the first sequential route in both groups is also a valid network ID in both block sizes.

Summarize routes 172.168.1.64/28, 172.168.1.80/28, 172.168.1.96/28 and 172.168.1.112/28 a single route 172.168.1.64/26 of block size 64.

Summarize routes 172.168.1.192/28 and route 172.168.1.208/28 into a single route 172.168.1.192/27 of block size 32.

Example of supernetting 4 (router A)

There are a total of 5 routes behind this router. Since the first two routes 0 and 48 do not have sequential routes,
we have to announce them individually. We cannot summarize a route that does not have a sequential route.

The remaining 3 routes 64, 72 and 80 are sequential with block size 8. The sum of the block sizes (8 + 8 + 8) is 24.
Since 24 is not a valid block size, we must exclude routes from the summary until the sum of the block sizes becomes equal
to a valid block size. If we exclude a route from the summary, the sum of the block sizes is reduced to 16, which is a valid block size.

In block size 16, 64 (the network ID of the first sequential route) is a valid network ID (0, 16, 32, 48, 64, 80 ……).

Summarize the first two routes in a summary route 122.128.58.64/28 of block size 16 and announce the third remaining route 122.128.58.80/29 independently.

Example of supernetting 4 (router B)

There are a total of 6 routes behind this router. Since the last route 56 has no sequential route, it cannot be summarized.

The other five channels 8, 16, 24, 32 and 40 are sequential. Total number of addresses in these
the routes are 40 (8 + 8 + 8 + 8 + 8). 40 is not a valid block size. The closest valid block size is 32.
So if we exclude a route (8 + 8 + 8 + 8-8 = 32), can we use block size 32 for the remaining routes?

No, even 32 is a valid block size, but it still cannot be used. To use it, the network ID of
the first route must be any form ID 0, 32, 64, 96, 128, 160, 192 and 224. In this case, the network ID for the first route is 8.
Thus, block size 32 cannot be used for synthesis.

Our next valid block size is 16. If we use this block size, we must create two summary routes and ignore one.
sequential route from the summary. Each summary route of block size 16 will summarize the 2 sequential routes of block size 8.

Since to use block size 16, we have to ignore one of the five sequences
routes and for the same reason explained above in block size 32, we cannot summarize the first route 8,
exclude the first route from the summary.

Summarize the remaining 4 routes (16, 24, 32 and 40) of block size 8 in two separate summaries
block size routes 122.128.58.16/28 and 122.128.58.32/28 16.

The following table lists the summary routes for the four routers.

Example 3 (router A) Example 3 (router B) Example 4 (router A) Example 4 (router B)
172.168.1.32/27 172.168.1.64/26 122.128.58.0/29 122.128.58.8/29
172.168.1.128/26 192.168.1.192/27 122.128.58.48/29 122.128.58.16/28
122.128.58.64/28 122.128.58.32/28
122.128.58.80/29 122.128.58.56/29

Itineraries that could not be summarized are formatted in bold and italics.

That’s it for this tutorial. If you have comments, suggestions, or comments on this tutorial,
please send me an email. If you like this tutorial, don’t forget to share it via your favorite social platform.

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