Route Summarization in Reverse

(Thanks to Bob Hennigan, CIS Instructor at Central New Mexico Community College, for this procedure.)

9 Jan 2014
kenglong@gmail.com

The Cisco CCNA exam will have several questions phrased like this.

What networks are included in this summary?

172 . 16 . 0 . 0 /12

Route summarization, as explained previously, is pretty simple. The concept is straight-forward and the procedure is easy. But, how do we go backwards? Given a summary route, how do we get the original routes? It turns out that the procedure to do so is also pretty easy.

First, determine what class the summary is in from the octet of interest. In this case, /12 indicates the second octet which is class B. We know the default mask for a class B is /16. Take the difference between the default mask and the mask of the summary.

16 - 12 = 4

Now raise 2 to the power of 4, the result of our subtraction, to get the number of original networks.

2⁴ = 16

So, there are 16 original networks. The summary mask, /12, plus the difference between the default and the summary masks, 4, is 16. Start with the summary address and count up by 16's.

172 . 16 . 0 . 0 /16, 172 . 16 . 16 . 0 /16, 172 . 16 . 32 . 0 /16, 172 . 16 . 48 . 0 /16, 172 . 16 . 64 . 0 /16 , 172 . 16 . 80 . 0 /16, 172 . 16 . 96 . 0 /16, 172 . 16 . 112 . 0 /16, 172 . 16 . 128 . 0 /16, 172 . 16 . 144 . 0 /16, 172 . 16 . 160 . 0 /16, 172 . 16 . 176 . 0 /16, 172 . 16 . 192 . 0 /16, 172 . 16 . 208 . 0 /16, 172 . 16 . 224 . 0 /16, 172 . 16 . 240 . 0 /16,

Pretty easy, huh? Let's do another one. Here's our new summary route.

192 . 168 . 0 . 0 /16

The octet of interest, third octet, indicates this is a class C.
The default mask for a class C is /24.
The difference between the default mask, /24, and the mask of the summary, /16, is 8.
2⁸ gives us 256 original networks.
Our network increment is 16 + 8 = /24, /24 = 1 in the third octet. Let's start counting.

192 . 168 . 0 . 0 /24 to 192 . 168 . 255 . 0 /24

CAUTION!

This procedure works well if the original network addresses all fall within a classful boundary. This procedure has problems if the original network addresses have been variably subnetted, VLSM!
In reality, you may never need to do reverse route summarization. In the CCNA exam, you might be given a summarized route with mask then asked to pick the original network addresses from a list of possible choices. This is much more reliable. All you have to do is calculate the range of original addresses from the summary then choose the answers that fall inside that range. (Thanks to Francis Nicholas for pointing this out to me.)

[end]