manual route summarization and supernetting

Posted on 2005-04-28
Last Modified: 2008-01-09
Is the following an example of manual route summarization.  Or is it an example of supernetting?

ps: If ripv2 or EIGRP were to summarize this route, it would advertise it as right

Question by:dissolved
    LVL 27

    Expert Comment

    Take the Octet you want to summarize and convert to binary.  Then figure out where they're all the same, and that's your "bit boundry".

    So, in your case it's:

    0      00000000
    1      00000001
    2      00000010
    3      00000011

    So, you take 6 bits plus the other 16 and you get the /22.

    I think supernetting and summarization are the same thing - summarizing networks by creating a supernet.

    Author Comment

    Thanks pseudocyber.

    1. So I put the octet of interest in binary, and compare. I find where they are similar.

    2. I count the number of bits that are similar (6 bits in our above example).

    3. Where do I go from here? Where did the "16" come from?
    LVL 79

    Assisted Solution

    Let me see if I can shed some light on your questions..

    Supernetting = using fewer mask bits than the "standard" classful mask to expand a network. For example, take a standard class C address, /24. Start running out of addresses and so instead of /24, you use /23 which just doubled your available hosts. That is supernetting. Problem is that not all TCP/IP stacks will recognize anything less than a standard classful mask. Many old printers, etc..

    Route summarization is just that, a summary of routes available through you. In your example, you know/have/own 4 Class C subnets and you broadcast to your next hop neighbor a single entry with /22 mask instead of 4 individual /24 entries. Since this is nothing more complicated than a route decision, little intelligence involved. Here, classful boundaries have no meaning. Consider these route entries:
    Any and all of them can be used as route summaries, only the Class C could be considered a supernet, if you must label it anything other than what it is - a route summay... /22 /22 /22


    Author Comment

    thanks! Finally I get the true answer of what supernetting is.

    whats the easiest way to manually summarize?
    I got halfway with pseudocyber's method and got stuck.

    Convert to binary, find what matches etc...
    0     00000000  
    1     00000001
    2     00000010
    3     00000011

    I understand where the 6 bits came from. But how did we get a /22 mask from this?
    LVL 27

    Accepted Solution

    The /16 comes from the OTHER two octets in the network portion.  The 192.168.  Then you're chopping the third octet in half.  So, those other two octets are 8 bits each.  8+8=16+6=22.

    Supernetting is the opposite of subnetting.  So, you can say to yourself, suppose I move the subnet bits some to the LEFT - then would the subnets I have fit inside the supernet I've created.

    So, if you have you could subnet it into two with and right, then, in effect, you could summarize those two with the first network IP - (because the range of hosts encompases your two subnets).

    Author Comment

    thanks a lot guys!

    Featured Post

    How your wiki can always stay up-to-date

    Quip doubles as a “living” wiki and a project management tool that evolves with your organization. As you finish projects in Quip, the work remains, easily accessible to all team members, new and old.
    - Increase transparency
    - Onboard new hires faster
    - Access from mobile/offline

    Join & Write a Comment

    Suggested Solutions

    Title # Comments Views Activity
    Cisco Routers 17 69
    Native Vlan Mismatch Impact 3 33
    MPLS VRF bridging 4 31
    Cisco layer 3 ring topology 1 22
    The Cisco RV042 router is a popular small network interfacing device that is often used as an internet gateway. Network administrators need to get at the management interface to make settings, change passwords, etc. This access is generally done usi…
    Tired of waiting for your show or movie to load?  Are buffering issues a constant problem with your internet connection?  Check this article out to see if these simple adjustments are the solution for you.
    After creating this article (, I decided to make a video (no audio) to show you how to configure the routers and run some trace routes and pings between the 7 sites…
    After creating this article (, I decided to make a video (no audio) to show you how to configure the routers and run some trace routes and pings between the 7 sites…

    729 members asked questions and received personalized solutions in the past 7 days.

    Join the community of 500,000 technology professionals and ask your questions.

    Join & Ask a Question

    Need Help in Real-Time?

    Connect with top rated Experts

    24 Experts available now in Live!

    Get 1:1 Help Now