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  • Summary Route

    Hello team,

    Can someone verify if my method of calculating summary route is correct. Or how would you do it.

    Here it goes.. I have the following subnets. and I need to do a summaray route on the router.

    172.16.0.0
    172.16.1.0
    172.16.3.0
    172.16.4.0
    172.16.6.0
    172.16.7.0
    172.16.8.0
    172.16.9.0
    172.16.10.0
    172.16.12.0
    172.16.13.0

    So to summary this route. I come up with 172.16.0.0/20. Here how I come up with that.
    I take the last(biggest subnet) 172.16.13.0 (I need 4 bits for 13)
    11111111.11111111.11110000.00000000

    so 8+8+4 = 20
    My assumption is 172.16.0.0/20 will summarize the given subnets.


    Is there another easy way to calculate summary route for subnet ranges?

    Thanks,
    HN

  • #2
    Re: Summary Route

    What you've got there works. But also bear in mind that your example doesn't list some subnets (2, 5, 11, 14, 15) in that range. In the summary, they will be assumed to be included in that route. If they are used elsewhere and don't follow the same routing path, you'll have problems getting traffic in the right direction. Just so you know.
    *RicklesP*
    MSCA (2003/XP), Security+, CCNA

    ** Remember: credit where credit is due, and reputation points as appropriate **

    Comment


    • #3
      Re: Summary Route

      Originally posted by RicklesP View Post
      What you've got there works. But also bear in mind that your example doesn't list some subnets (2, 5, 11, 14, 15) in that range. In the summary, they will be assumed to be included in that route. If they are used elsewhere and don't follow the same routing path, you'll have problems getting traffic in the right direction. Just so you know.

      I hear what you are saying.. but let say I have 172.16.0.0-.13.0
      My confusion is how or what is the right way to calculating the route summary manually? what is the best way doing it. Im not sure if my method is correct.

      Comment


      • #4
        Re: Summary Route

        Think like you would when sub- or supernetting.

        A summary route is just like a supernet covering all the networks you want to summarize, and since you only have 9 different possible values for each octet in the netmask, choosing the right summary value should be easy.

        You didn't include the size of each network to be covered (a rather important piece of information), but I'll assume they were all /24 subnets.

        While 172.16.0.0/24 covers just the one /24 network (the third octet of the netmask is 11111111 in binary). 172.16.0.0/23 covers 172.16.0.0/24 and 172.16.1.0/24, 172.16.0.0/22 covers 172.16.0.0/24 to 172.16.3.0/24and so on:

        Code:
        Summary         Value of the third         Networks
        address/mask:   netmask octet in binary:   Covered:
        172.16.0.0/24   11111111                   172.16.0.0
        172.16.0.0/23   11111110                   172.16.0-1.0
        172.16.0.0/22   11111100                   172.16.0-3.0
        172.16.0.0/21   11111000                   172.16.0-7.0
        172.16.0.0/20   11110000                   172.16.0-15.0
        172.16.0.0/19   11100000                   172.16.0-31.0
        172.16.0.0/18   11000000                   172.16.0-63.0
        172.16.0.0/17   10000000                   172.16.0-127.0
        172.16.0.0/16   00000000                   172.16.0-255.0
        
        A summary route works like a network in that the number of addresses/subnets/networks it covers is always a power of two, but just like subnets, they also cover an address range starting at that same power of 2. You can cover 0-1 or 4-5 using one /23 summary address, but the only summary address covering both 172.16.127.0/24 and 172.16.128.0/24 is 172.16.0.0/16.

        As RicklesP said, the summary address covers several networks not in the list. That's not a problem, since a specific route always trumps a more general one. A route to 172.16.2.0/24 will always override a summary route to 172.16.0.0/20.
        Last edited by Ser Olmy; 13th March 2014, 23:23.

        Comment


        • #5
          Re: Summary Route

          Thanks Gens.

          okay.. that is very good layout Ser Olmy. I guess my assumption was correct. If someone give me a range off networks and have that route summarize, I can just take the last(or biggest) route, then mask the bits, and I should be good.

          RicklesP and Ser Olmy, it was a typo on my end that I didn't list all the routes.

          Subnets is not my strong point, I'm working to get better at it. I understand that there is a quick way to use online subnet/route calculating etc.. But I just want to have a good understanding the method behind. I want to able to explain to someone if they ask me how does the route summarization or subnets working, especially if these kind of questions asked during the job interview. I hate to tell them "ohhhhhhhhhhh you can go to subnetDOTcom and you can get your answer in 30secs "

          Thanks,
          HN
          Last edited by Humannetwork; 14th March 2014, 02:20.

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