Route Summarization

Hi Rene,

Regarding the below example , how are you determining that the block size is 64? Also 172 is a class B address. How it is having /24 which is a class C address.?
Can you pls explain?

Classful addressing is considered out of date meaning that nobody cares about it any more. So while technically 172.x.x.x used to be called Class B, there is no reason you can’t further subnet it–this is called Variable Length Subnet Mask (VLSM). In modern networks, class A vs B vs C really doesn’t mean anything anymore.

Now, as far as why the block size in your example is 64 … If you read my post about two above yours ----^ I gave an explanation on how I personally do subnetting. So, let’s apply what I said there to your example.

Start by determining what size our subnet needs to cover. In your case, you want the third octet to range from 10 - 50. This means the whatever block size we come up with has to be at least 40 (50 minus 10). Now, let’s apply the formula I talked about: 2^x >= 40, so that reads: “two to the Xth power is greater than or equal to 40.” Solve for X …

If we choose X = 5, we know that is too small because 2 to the 5th power = 32.
If we choose x = 6, we know this is big enough, because 2 to the 6th power = 64.

This is how we came up with 64. The summary address would be, or

Thanks, Andrew. But Rene had mentioned the summary address as Can you pls clarify here?

Oops! I made a mistake. The calculation of 6 (which means 6 bits are required for the summarization) is correct, but I applied those six bits to the “host” portion of the subnet mask and not the “network” portion. I have corrected my original post–thanks for pointing that out.

Hi Rene,

Would appreciate if you could bring out the difference between CIDR, Summarization and Supernetting.

CIDR and Supernetting are the same thing. This is where you can aggregate networks together into larger networks beyond their natural network boundary.

Although Supernetting/CIDR might be considered a type of summarization, “summarization” generally is considered to be constrained by natural (classful) network boundaries. The distinction is somewhat subtle. Suppose you have the following:

These could be summarized as because /27 is smaller than the natural /24 boundary.

Now, if you had this:

The supernet (or CIDR) of these would be because /23 is greater than the natural /24 boundary.

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19 posts were merged into an existing topic: Route Summarization

2 posts were merged into an existing topic: Subnetting in Decimal (Fast Way)

Hello Rene, Thanks for explaining the Summarization in a simple way.

I have a question regarding the example you used.
Let’s look at another example. Let’s say we want to summarize the following networks: / 16 subnet mask / 16 subnet mask / 16 subnet mask / 16 subnet mask / 16 subnet mask / 16 subnet mask / 16 subnet mask / 16 subnet mask

using the binary method it is clear that the subnet mask is 255.248 or in CIDR /13 i.e.
The summary address will be /13 (subnet mask will be
using the number of networks also it is clear as 8 networks so 256 - 8 is 248

However when we use the block size method .

128 64 32 16 8 4 2 1
then block size 32 is covering all the networks and 256 - 32 = 224 so the subnet is coming as and the range is - and all 8 networks are coming in this range.

where as with 8 block size we can have network from - and it is not covering all networks.

also, want to know if my understanding is not correct then on what section should i go back to study it more.

Also, why you are consider summary as / 13

as the changes are happening in the second octet and first octet is not changing so is the summary below is correct ??? / 11

Hello Andrew,

Thanks for explaining difference between Supernetting and summarization.
but still having confusion and not able to grasp it completely.
Is is possible if you can explain with some examples and using the same example both summarization and supernetting. like for
what will be summarization and what will be supernetting.

Hello Tejpal

For this particular example, the number of networks is once again 8, not 32. We are looking at it from the point of view of the second octet and not the third as is the case in the previous example. So using 8, we have 256-8 = 248, so the subnet mask would be, once again, focusing on the second octet. This would result in or which encompasses IP addresses between to

An 8 block size will give us a subnet mask of which will cover networks from

  • to
  • to
  • to
  • etc

Notice that the third point is indeed the subnet we are looking for.

Actually, Rene states the following:

The summary address will be /13 (subnet mask will be

I hope this has been helpful!


Hello Tejpal

Supernetting and summarization are similar operations, but are applied differently to networks. If you have two networks such as the ones you mentioned in your post, the summarization of the two will be: This represents two separate and distinct networks as they are shown in your post. Now this can be used in a routing table to represent these two networks with a single routing table entry.

Supernetting on the other hand is the actual concatenation of networks, that is, changing the configuration of the devices found in these two subents so that the two sets of devices are no longer on two different subnets but on one subnet, the supernet of the two. So devices in such a network will change their subnet masks from /28 to /27 and any other default gateway addresses that may need changing as well.

So summarization is the representation of multiple networks with a single statement.
Supernetting is the act of actually concatenating multiple existing subnets into a single subnet or supernet.

I hope this has been helpful.

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OK, Thanks Lazaros for your reply and it helps me understanding it in a more better way. :blush:
Please let me know if my answer is correct regarding the explanation give below :
so, if i understand correctly then the reason as why the summary address used is
and not or or any other network because if we use the subnet /13 then
the network starts with and the first usable IP address is and the last usable IP address is and broadcast address
Please let me know if my above statement is true for selecting as the summary address.

Thanks, Lazaros. Just want to clear one more thing regarding the subnet -

Please let me know the order in which IP address increase for this subnet is correct.,, , ( first the value of 4th Octet will increase and once it becomes
255 the value of 3rd octet will increase from 0 to 1), ,,, 172,16,2,2 ,,,, 172,16,3,3 , 172,16,253.255,,, --------------- (now the 4th and 3rd octet becomes 255 then the
2nd octet value changed to 17 from 16 )
,,, ,172.23.151, 155,,,,

so first 4th octet value increases and then once it becomes 255 then the third octet value increases
and when both 4th and 3rd octet Values become 255 then the 2nd octet value increases.
Please, guide if the above statement and the IP addresses are used in the correct order.

Sorry Lazaros, disturbing again but can you explain how summarization is for /28

as with 16 block size i.e. with subnet /28 also it will cover network -, -, -,

so did not know why you take 32 block size and subnet as /27 for the summarization

Hello Tejpal

Yes, you are absolutely correct. It looks like you’ve grasp the concept well!


Hello Tejpal

Yes, you are correct once again. Just a small little issue, after comes, etc… Don’t forget the 0 values in the octets as well!

I hope this has been helpful!


Hello Again Tejpal

The /28 prefix provides 16 addresses while the /27 provides 32. The two address spaces of and together make a single /27 address space. This means that both can fit into the summary.

If you look at the summary, you’ll see that the first address is and the last is which encompasses both subnets.

I hope this has been helpful!


How come we do not use a block size of 4?

Hello Rick

A block size of four would cover the addresses first four networks, that is -, but here we have five blocks of /24 addresses. In order to cover this we need a block of eight, to include that one extra address block.

I hope this has been helpful!


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