Supernetting Tutorial: - Supernetting Explained with Examples

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

What is Supernetting?

Supernetting is the process of summarizing a bunch of contiguous Subnetted networks back in a single large network. Supernetting is also known as route summarization and route aggregation.

Following figure shows an example of Supernetting.

In above example, 8 subnets are summarized in single subnet.

Why Supernetting is done?

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

Without Supernetting, router will share all routes from routing tables as they are. With Supernetting, it will summarize them before sharing. Route summarization reduces the size of routing updates dramatically.

Following figure shows an example of route summarization.

Advantage of Supernetting

Supernetting provides following advantages.

• It reduces the size of routing updates.
• It provides a better overview of network.
• It decreases the use of resources such as Memory and CPU.
• It decreases the required time in rebuilding the routing tables.

Supernetting components

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

In order to perform the Supernetting, we need Network ID, CIDR Value, Broadcast ID, Subnet Mask and Block Size of each route.

• Network ID and broadcast ID are used to check the alignment of routes. Supernetting can be performed only if routes are sequential.
• Block size is used to calculate the summarized route from given routes.
• Subnet mask and CIDR value is the same thing in different notations. Both are used to find the ON network bits in IP address. In exam, question may use any notation. While preparing for Cisco exam, you should practice with both.

Since an advertise route is the combination of network ID and CIDR value, we only need to figure out the broadcast ID, subnet mask and block size.

For block size use following formulas:-

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

For example if CIDR value is 25 then block is 128.

32 -25 = 7
27 = 128

Broadcast ID is the last address of network. Once you know the block size, to calculate the broadcast ID, simply count the addresses starting from network ID till the last address of the block.

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

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

Following table lists all CIDR values along with subnet mask and block size.

Supernetting chart

Key points of Supernetting

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

A route can be summarized only in a route which is bigger than it in block size. For example we can’t summarize a route of block size 64 in a route of block size 32 but we can summarize two routes of block size 32 in a single route of block size 64.

The easiest way of calculating the summarized route is adding the block size of all sequential routes and using the Subnetting which provides the block size that is equal to the result of addition. For example if we have two sequential routes of block size 16, we can summarize them in a single route of block size 32.

Summarization can be done only in available bock sizes. For example if we have 5 routes of block size 8, we cannot summarize them in single route of block size 40 (8x5). 40 is not a valid block size. For valid block sizes see the Supernetting chart give above. In this case, the best choice is summarizing first four routes is single summarized route of block size 32 and keeping the fifth route as it is.

Just like block size, network ID of summarized route must be matched with the network ID of first sequential route. To calculate the valid network in summarized block size, simply count in block size starting from 0.

For example, if summarized block size is 32 then valid network IDs are 0, 32, 64, 96, 128, 160, 192 and 224. If the first sequential route of routes which we are summarizing doesn’t start with any one of these network IDs, they can’t be summarized in a single route of block size 32, even they satisfy the block size requirement.

For instance the route 192.168.1.16/28 and the route 192.168.1.32/28 can’t be summarized in a single route of block size 32 even they are sequential and their collective block size (16+16) is equal to the 32.

Any sequential routes which start with any one of these network IDs can be summarized with this block size. For instance route 192.168.1.0/28 and the route 192.168.1.16/28 can be summarized in a single route 192.168.1.0/27 of block size 32

Never select a block size which does not cover all addresses unless it is clearly mentioned in question that remaining addresses will be used behind the router where summarization will be performed.

For example, if we have two routes with block size of 16 and 8, we can’t summarize them in a single route of block size 32. If we do that, router will advertise a summarized route that says this router have network path for 32 addresses while in reality it have network path only for 24 (16+8) addresses.

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

Supernetting Examples Explained Step by Step

Above we took the two examples of Supernetting. Let’s understand how Supernetting was performed in them step by step.

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

Write the CIDR value, Subnet Mask, Network ID, Broadcast ID and block size of each route.

Supernetting Example 1

Supernetting Example 2

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

Add the block size of all sequential routes.

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

Check the nearest valid block size which provides equal or less number of addresses. The block size 256 and 1024 exactly match with our requirement. The Subnetting of /24 and /22 give us the block size of 256 and 1024 respectively.

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

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

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

Complex Supernetting Examples

If IP addressing is planed correctly, Supernetting is simple and straightforward as we have seen in above examples. It becomes difficult only if unplanned IP addressing is used in network.

If you are preparing for Cisco exam, you should practice with unplanned IP addressing. To test candidates’ caliber, Cisco usually puts complex and unplanned networks in Supernetting related questions.

To get an overview of how Supernetting questions could be difficult in Cisco exam, let’s have two examples of complex Supernetting. These examples are based on Supernetting questions asked in CCNA Routing and Switching exam.

Supernetting Example 3

Supernetting Example 4

Step 1: - Arrange all routes in ascending order.

Step 2: - Write the network ID, broadcast ID, CIDR value, subnet mask and block size of each route.

Supernetting Example 3 (Router A)

Supernetting Example 3 (Router B)

Supernetting Example 4 (Router A)

Supernetting Example 4 (Router B)

Step 3: - Based on network ID and Broadcast ID make the group of sequential routes.

Step 4: - Summarize each group of sequential routes in a single or multiple summarized routes.

• Add the block size of all sequential routes in group.
• Find the valid block which is equal or less in size.
• Starting from 0, count in valid block size and check whether the network ID of first sequential route exists in result or not.
• If network ID of first sequential route exists in result, use the valid block size to summarize the routes.
• For summarization use the CIDR value which provides this block size.
• If network ID of first sequential route does not exist in result, use smaller valid block size and count again. Repeat this step until the network ID of first sequential route does not fall in result.

Supernetting example 3 (Router A)

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

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

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

In 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. Network ID of first sequential route (128) is also in the range of block size 64 (0, 64, 128, 192 ……). Thus, we can use the block size 64 to summarize these routes. CIDR value of block size 64 is /26. Let’s use it to summarize these routes.

Summarize the 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.

Supernetting example 3 (Router B)

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

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

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

Supernetting example 4 (Router A)

There are total 5 routes behind this router. Since first two routes 0 and 48 have no sequential routes, we have to advertise them individually. We can’t summarize a route which has no sequential route.

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

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

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

Supernetting example 4 (Router B)

There are total 6 routes behind this router. Since last route 56 has no sequential route, it can’t be summarized.

Remaining five routes 8, 16, 24, 32 and 40 are sequential. Total numbers of address in these routes are 40 (8+8+8+8+8). 40 is not a valid block size. The nearest valid block size is 32. So if we exclude one route (8+8+8+8-8 = 32), can we use the block size 32 for remaining routes?

No, even 32 is a valid block size, still it can’t be used. In order to use it, network ID of first route must be any one ID form 0, 32, 64, 96, 128, 160, 192 and 224. While in this case, network ID of first route is 8. Thus the block size 32 can’t be used for summarization.

Our next valid block size is 16. If we use this block size, we have to create two summarized routes and skip one sequential route from the summarization. Each summarized route of block size 16 will summarize the 2 sequential routes of the block size 8.

Since in order to use the block size 16, we have to skip one route from the five sequential routes and due to the same reason explained above in block size 32 we can’t summarize the first route 8, exclude the first route from summarization.

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

Following table lists the summarized routes for all four routers.

The routes which couldn’t be summarized are formatted in bold and italic.

That’s all for this tutorial. If you have any feedback, suggestion or comment about this tutorial, please mail me. If you like this tutorial, don’t forget to share it through your favorite social platform.