Supernetting Tutorial: - Supernetting Explained with Examples

Supernetting is also known as route summarization or aggregation. It is the opposite of subnetting. In subnetting, we break a single subnet into many subnets. In supernetting, we join multiple subnets into a single subnet.

The network address and block size are two main elements of supernetting. We can perform supernetting only on the subnets having the same network address. If subnets have different network addresses, we cannot combine them through the supernetting.

When matching network addresses, we need to take care of two things. The default network address of all subnets must be the same. The default network address is the address we get from default network bits. The following table lists the default network and host bits.

If the default network address is the same in all subnets, we can summarize them in a subnet having the same network address. Let us take an example. We have four class C subnets.

192.168.1.0/26, 192.168.2.64/26, 192.168.3.128/26, and 192.168.4.192/26.

In class C, the network bits are the first 24 bits (or three octets). These bits must be the same in all subnets.

192.168.1.0/26, 192.168.2.64/26, 192.168.3.128/26, and 192.168.4.192/26.

Since these subnets have different default network addresses, we can not summarize them in a single subnet.

Let us take another example. We have four subnets.

192.168.0.0/26, 192.168.0.64/26, 192.168.0.128/26, and 192.168.0.192/26.

These subnets have the same network address (192.168.0). We can summarize them in a single subnet 192.168.0.0/24. This subnet also has the same network address (192.168.0).

Block size

The second key element is the block size. To summarize subnets, we use a subnet having a block size equal to or more than the sum of the block sizes of all subnets.

In the preceding example, we have four subnets having a block size of 64. To summarize these subnets, we need a subnet of a block size of 256 (64 x 4 = 256). The block size of the subnet 192.168.0.0/24 is 256. Hence, we can use it to summarize these subnets.

Route summarization steps

• In the first step, we check the network address of all subnets. If default network bits are the same in all subnets, we find the block size of all subnets.
• In the second step, we add the block sizes of all contiguous subnets. We can summarize only contiguous networks.
• In the third step, we find a subnet that has the same network address and the block size equal to or greater than the sum of all block sizes.

Route summarization example1

192.168.1.0/25, 192.168.1.128/26, 192.168.1.192/27, 192.168.1.224/28, 192.168.1.240/30, 192.168.1.244/30, 192.168.1.248/30, 192.168.1.252/30

Route summarization example2

10.0.0.0/23, 10.0.2.0/24, 10.0.3.0/25, 10.0.3.128/26, 10.0.3.192/27, 10.0.3.224/28, 10.0.3.240/30, 10.0.3.244/30, 10.0.3.248/30, 10.0.3.252/30

In the first step, we arrange all subnets in ascending order based on their CIDR values. If the CIDR value of two subnets is the same, we use their network addresses. We also write the network and broadcast addresses of all subnets. We use them to identify contiguous subnets.

Supernetting Example 1

Supernetting Example 2

In the second step, we add the block sizes of all contiguous networks.

Network1 = 128 + 64 + 32 + 16 + 4 + 4 + 4 + 4 = 256
Network2 = 512 + 256 + 128 + 64 + 32 + 16 + 4 + 4 + 4 + 4 = 1024

In the third step, we find the subnet having a block size equal to or greater than the sum of the block sizes of all contiguous subnets in the block size chart. The following table lists the block sizes of all IP subnets.

In the first example, the sum of all subnets' block sizes is 256. The block size of CIDR /24 is 256.

In the next step, we count in the multiple of the block size. The network number of the first subnet must exist in the range.

192.168.0.0/24, 192.168.1.0/24, 192.168.3.0/24.......

If the network number of the first subnet exists in the range, we use this CIDR with the network address of the first subnet to summarize the contiguous subnets. Hence, the 192.168.0.0/24 is the summarized subnet of the first example.

In the second example, the sum of all subnets' block sizes is 1024. The block size of CIDR /22 is 1024.

10.0.0.0/22, 10.0.4.0/22, 10.0.8.0/22 ....

The network address of the first subnet is 10.0.0.0. Therefore, 10.0.0.0/22 is the summarized subnet of the second example.

Route summarization on non-contiguous subnets

If a network contains non-contiguous subnets, we can not summarize all subnets in a single subnet. In that case, we can summarize contiguous subnets and leave non-contiguous subnets untouched.

Let us take two more examples.

Route summarization example 3

Route summarization example 4

Let us perform route summarization for the above subnets.

In the first step, we arrange all subnets in ascending order based on their block sizes. We also write their network and broadcast addresses.

Route summarization 3 (Router A)

Route summarization 3 (Router B)

Route summarization 4 (Router A)

Route summarization 4 (Router B)

In the second step, we group all contiguous subnets. We leave the uncontiguous networks.

In the third step, we find the subnet having a block size equal to or greater than the sum of the block sizes of all contiguous subnets.

In the fourth step, we count in the block size. If the network address of the first subnet is in the range, we use the summarized subnet.

If it is not in the range, we use the next available block size in descending order in the block size chart and count again. We repeat the process until the network address of the first subnet does not come in the counting range of the summarized route's block size.

Supernetting example 3 (Router A)

On Router A, we have two groups of contiguous subnets.

In the first group, we have two contiguous subnets: 32 and 48. Both have a block size of 16. The sum of block sizes is 32 (16+16). 32 is a valid block size. A valid block size is a block size that is available in the block size chart.

The network ID of the first sequential subnet is 32, which is a valid network ID in block size counting (0, 32, 64,…). Block size 32 is associated with CIDR value /27. Hence, we will use this block size for summarization.

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

In the second group, we have four sequential subnets: 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 range of block size (64) counting (0, 64, 128, 192 ……). Thus, we can use the block size 64 to summarize these subnets. The CIDR value of block size 64 is /26. We will use it to summarize these subnets.

Summarize the subnets 172.168.1.128/28, 172.168.1.144/28, 172.168.1.160/28 and 172.168.1.176/28 in a single subnet 172.168.1.128/26 of block size 64.

Supernetting example 3 (Router B)

This router also has two groups of contiguous subnets.

In the first group, we have four contiguous subnets: 64, 80, 96, and 112 of block size 16. The sum of block sizes is 64 (16+16+16+16). 64 is a valid block size. If we count in block size, the network address of the first subnet is also available in the list.

172.168.1.0/26, 172.168.1.64/26, 172.168.1.128/26....

Therefore, we can summarize the subnets 172.168.1.64/28, 172.168.1.80/28, 172.168.1.96/28, and 172.168.1.112/28 in a single subnet 172.168.1.64/26 of block size 64.

In the second group, we have two contiguous subnets: 192 and 208 of block size 16. The sum of block sizes is 32 (16+16). 32 is a valid block size. If we count in block size, the network address of the first subnet is also available in the list.

172.168.1.0/27, 172.168.1.32/27, 172.168.1.64/27, 172.168.1.96/27, 172.168.1.128/27, 172.168.1.160/27, 172.168.1.192/27, 172.168.1.224/27....

Therefore, we can summarize the subnets 172.168.1.192/28 and 172.168.1.208/28 in a single subnet 172.168.1.192/27 of block size 32.

Supernetting example 4 (Router A)

On this router, we have five subnets. The first two subnets (0 and 48) have no contiguous subnets. Hence, we cannot summarize them. The router will advertise them individually.

The remaining three subnets (64, 72, and 80) are contiguous. The sum of block sizes (8+8+8) is 24. 24 is not a valid block size. It is not available in the block size chart. We can use only the block sizes available in the block size chart.

If we exclude one subnet, the sum of block sizes (8+8) will be 16. 16 is a valid block size.

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

Therefore, we can summarize the subnets 122.128.58.64/29 and 122.128.58.72/29 in a single subnet 122.128.58.64/28 of block size 16.

We cannot summarize the third subnet even if it is contiguous. The router will advertise it independently.

Supernetting example 4 (Router B)

On this router, we have six subnets. The last subnet (56) has no contiguous subnets. Hence, we can summarize it.

The remaining five subnets (8, 16, 24, 32, and 40) are contiguous.

The sum of block sizes (8+8+8+8+8) is 40. 40 is not a valid block size. It is not available in the block size chart. We can use only the block sizes available in the block size chart.

If we exclude one subnet, the sum of block sizes (8+8+8+8) will be 32. 32 is a valid block size.

The following are the valid network addresses in block size 32.

122.128.58.0/27, 122.128.58.32/27, 122.128.58.64/27......

Since the network ID of the first contiguous subnet is not available in the above list, we cannot use this block size. The next smaller block size is 16. The valid network addresses of block size 16 are the following.

122.128.58.0/27, 122.128.58.16/27, 122.128.58.32/27, 122.128.58.48/27, 122.128.58.64/27, 122.128.58.80/27, 122.128.58.96/27......

The network addresses of the second (122.128.58.16/29) and fourth (122.128.58.32/29) subnets are available in the above list. We can use this block size to summarize the subnets.

We will summarize the subnets 122.128.58.16/29 and 122.128.58.24/29 in the subnet 122.128.58.16/27 of a block size of 16.

We will summarize the subnets 122.128.58.32/29 and 122.128.58.40/29 in the subnet 122.128.58.32/27 of a block size of 16.

The following table lists the summarized subnets for all four routers.

The subnets which we can not summarize are listed in bold and italics.

That's all for this tutorial. In this tutorial, we learned route summarization through examples.