08
Aug
08

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:
192.168.1.0/26 (64hosts: 192.168.1.0-192.168.1.63)
192.168.1.64/26 (64hosts: 192.168.1.64-192.168.1.127)
192.168.1.128/26 (64 hosts: 192.168.1.128-192.168.1.191)
192.168.1.192/26 (64 hosts: 192.168.1.192-192.168.1.255)

which goes through single router to Internet. Then we can summarize the route to:
192.168.1.0/25 (128 hosts: 192.168.1.0-192.168.1.127)
192.168.1.128/25 (128 hosts: 192.168.1.128-192.168.1.255)

We further summarize it to 192.168.1.0/24 (256hosts: 192.168.1.0-192.168.1.255)

Quite often we make mistake summarizing routes, by using the following example, here we got 2 route:
192.168.1.64/26 (64hosts: 192.168.1.64-192.168.1.127)
192.168.1.128/26 (64 hosts: 192.168.1.128-192.168.191)
we might summarize them to 192.168.1.64/25, which is INCORRECT, because 192.168.1.64/25 have the IP addresses starting from 192.168.1.0 – 192.168.1.127, NOT 192.168.1.64 – 192.168.1.191.

*192.168.1.64/25 doesn’t mean 192.168.1.64 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 192.168.1.64/25.

Take the magic number 32, minus the slash notation (25):
32-25=7

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 (192.168.1.64), 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. 192.168.1.0/29 (8 hosts: 192.168.1.0-192.168.1.7)
2. 192.168.1.8/29 (8 hosts: 192.168.1.8-192.168.1.15)
3. 192.168.1.40/29 (8 hosts: 192.168.1.40-192.168.1.47)
4. 192.168.1.48/29 (8 hosts: 192.168.1.48-192.168.1.55)

We can summarize

Route 1 and 2 into:
A. 192.168.1.0/28 (16 hosts: 192.168.1.0-192.168.1.15)

Route 3 and 4 into:
B. 192.168.1.40/28 (16 hosts: 192.168.1.40-192.168.1.55)

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

Let check the validity of the route A 192.168.1.0/28 (first IP address: 192.168.1.0):
32-28=4
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 192.168.1.40/28 (first IP address: 192.168.1.40):
32-28=4
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:

192.168.1.0/28
192.168.40/29
192.168.48/29

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.

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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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