Route Summarization (CCNA Tips)

When we come to router configuration, it is often required to do route summarization for the subnets on the LAN.

By using route summarization, it helps:

  • reduce the size of routing table held by the router
  • reduce the need to of route updates
  • overcome/neglect network problem (flipping network)

In short, it is an add-up of various narrow routes into wide route(s). By example, we have several subnet in LAN of: (64hosts: (64hosts: (64 hosts: (64 hosts:

which goes through single router to Internet. Then we can summarize the route to: (128 hosts: (128 hosts:

We further summarize it to (256hosts:

Quite often we make mistake summarizing routes, by using the following example, here we got 2 route: (64hosts: (64 hosts:
we might summarize them to, which is INCORRECT, because have the IP addresses starting from –, NOT –

* doesn’t mean is the starting address, you can just put any address as you like (Yes! Any address as you like, because it doesn’t necessary¬† to be starting address, however for ease of calculation, we encourage first IP to be used).

Here comes a little tip to prevent mis-summarized route, by little calculation. Let’s take the previous incorrect summarized route

Take the magic number 32, minus the slash notation (25):

Make it the power of 2:
2 the power of 7 = 128 (2x2x2x2x2x2x2)

Take the last octect from first ip address of the summarized route (, divided by 128:
64/128 = 0.5

If it is a round number (0, 1, 2, 3…), the route summarization is correct, vice versa!

Let’s have some fun. For instance:

1. (8 hosts:
2. (8 hosts:
3. (8 hosts:
4. (8 hosts:

We can summarize

Route 1 and 2 into:
A. (16 hosts:

Route 3 and 4 into:
B. (16 hosts:

Route A and B can’t be further summarized, because they are not contiguous!

Let check the validity of the route A (first IP address:
2 the power of 4 = 16 (2x2x2x2)
0 (last octect) / 16 = 0 (round number!)

So route A is valid!

Let check the validity of the route B (first IP address:
2 the power of 4 = 16 (2x2x2x2)
40(last octect) / 16 = 0.25 (not round number!)

So route B is invalid!

At the last, the route summarization of these 4 route are:

When you are not sure if your route summarization works, just simply make a count.

* For slash notation < 24, just simply ignore the last octet
Change the magic number to 24, take second last octet in calculation.

* For slash notation <16, we just simply ignore the last and second last octet.
Change the magic number to 16, take the second octet in calculation.


9 Responses to “Route Summarization (CCNA Tips)”

  1. 2 bels
    November 11, 2009 at 12:53 pm

    thanks but you dont show how to do it!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

  2. 3 bels
    November 11, 2009 at 12:53 pm

    dont you wonder y thez only been one comment ???? duh

    • 4 albertsiow
      November 12, 2009 at 7:37 am

      Hi, Bels,
      I am not quite sure what you want to know in further detail?
      This blog is just my personal backup for technical solution/finding, and the focus of this post is soley on performing mathematical calculation with various subnet, in order to understand route summarization concept in CCNA syllabus.
      I am more that happy if you let me know what you wanna know more. Thanks!

  3. 6 sukanta
    March 16, 2012 at 7:39 am

    thanks dear………
    u cleared my confusion and now i have a good concept ….. thanks……

  4. 7 raymund
    January 9, 2013 at 12:42 pm

    do u have any summarization in CCNA 2 chpters 1 to 6

  5. July 11, 2013 at 9:12 am

    I am now not sure the place you are getting your information,
    however good topic. I needs to spend some time studying much
    more or understanding more. Thank you for great info I used to be in search of this info for my mission.

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