Answers: Summary Route Practice Set 1

certskills
By certskills February 21, 2014 09:05

#CCNA (and #CCENT) candidates, here are the answers to that earlier summary route practice set. If you’ve not tried these problems yet, go back, check it out, and come up with the best summary route. Then come back here and check your answers.

The answer to each of the three problems uses two tables. The first table lists the original masks and subnet IDs of the routes that need to be summarized as one route, along with the calculated subnet broadcast addresses. These tables also show the low and high end of the address range in total, with bold text for the lowest subnet ID (the low end of the range) and bold text for the highest subnet broadcast address (the high end of the range). The best summary route will be the first subnet/mask combination found that includes both the low and high-end numbers.

The second table for each answer lists the calculation of potential correct answers, with the correct answer listed in bold. The table shows each successive calculation of a new subnet ID, using a new /P mask, and its broadcast address. (See this post for details about the process.) The table shows the calculations with each /P being 1 smaller than the previous, until it lists the correct answer.

Note that the second table for each problem includes a final summary route with a mask (/P) one shorter than the correct best summary route. This last route also includes the entire range of addresses, and could be used as a summary, but is not considered the best summary route, because it includes an unnecessarily broad range of addresses.

 

Problem 1: Answers

The following table lists the original masks and subnets, the calculated broadcast addresses for each, and notes the lowest and highest numbers in the range in bold.

 

Problem 1, Original Subnets and Broadcast Addresses

Mask Subnet Broadcast Address
/16 10.100.0.0 10.100.255.255
/16 10.102.0.0 10.102.255.255
/16 10.104.0.0 10.104.255.255
/16 10.101.0.0 10.101.255.255

If you are using the literal process shown in aforementioned post about how to find summary routes, the next table shows the results of working repeatedly through step 4. If not… the table shows a calculation, using progressively smaller masks, calculating a new subnet ID and matching broadcast address, using the smallest original subnet ID.

Note that the bold entry in the table is the correct answer.

 

Problem 1, Calculation Using 10.100.0.0, New /P

Mask Calculated Subnet Broadcast Address
/15 10.100.0.0 10.101.255.255
/14 10.100.0.0 10.103.255.255
/13 10.96.0.0 10.103.255.255
/12 10.96.0.0 10.111.255.255
/11 10.96.0.0 10.127.255.255

 

Problem 2: Answers

The following table lists the original masks and subnets, the calculated broadcast addresses for each, and notes the lowest and highest numbers in the range in bold.

 

Problem 2, Original Subnets and Broadcast Addresses

Mask Subnet Broadcast Address
/26 172.16.207.192 172.16.207.255
/26 172.16.205.64 172.16.205.127
/25 172.16.206.128 172.16.206.255
/24 172.16.204.0 172.16.204.255

If you are using the literal process shown in aforementioned post about how to find summary routes, the next table shows the results of working repeatedly through step 4. If not… the table shows a calculation, using progressively smaller masks, calculating a new subnet ID and matching broadcast address, using the smallest original subnet ID.

Note that the bold entry in the table is the correct answer.

 

Problem 2, Calculation Using 172.16.204.0, New /P

Mask Calculated Subnet Broadcast Address
/23 172.16.204.0 172.16.205.255
/22 172.16.204.0 172.16.207.255
/21 172.16.200.0 172.16.207.255

 

Problem 3: Answers

The following table lists the original masks and subnets, the calculated broadcast addresses for each, and notes the lowest and highest numbers in the range in bold.

Problem 3, Original Subnets and Broadcast Addresses

Mask Subnet Broadcast Address
/30 192.168.1.100 192.168.1.103
/29 192.168.1.80 192.168.1.87
/30 192.168.1.72 192.168.1.75
/28 192.168.1.112 192.168.1.127

If you are using the literal process shown in aforementioned post about how to find summary routes, the next table shows the results of working repeatedly through step 4. If not… the table shows a calculation, using progressively smaller masks, calculating a new subnet ID and matching broadcast address, using the smallest original subnet ID.

Note that the bold entry in the table is the correct answer.

 

Problem 3, Calculation Using 192.168.1.72, New /P

Mask Calculated Subnet Broadcast Address
/27 192.168.1.64 192.168.1.95
/26 192.168.1.64 192.168.1.127
/25 192.168.1.0 192.168.1.127
Summary Route Practice Set 1
Summary Route Practice Set 2
certskills
By certskills February 21, 2014 09:05
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6 Comments

  1. adriana November 17, 13:20

    Hello Wendell !
    Recently I have been working on route summarization and I have found several post on the web … Your post has been very useful but I have some doubts about trying to summarize some IP ranges that are not contiguous.
    For example, in the first problem in your post it is needed to summarize addresses from 10.100.0.0 to 10.102.255.255 and then another rage of contiguous IP address from 10.104.0.0 to 10.104.255.255.
    Trying to summarize only those IP ranges I think there are needed three different and separate ranges: 10.100.0.0/15, 10.102.0.0/16 and 10.104.0.0/16
    The solution you propose include a too many other addresses not included in original IP ranges.
    Would you mind to explain why is 100.96.0.0/12 the right answer? This range includes 1048576 addresses in total, while the original IP ranges include only 4 ranges with 65536 addresss in total having 262144 addresses in all the 4 ranges.

    Thanks a lot !!!

    Reply to this comment
    • CCENTSkills November 18, 10:30

      Hi Adriana,
      Thanks for the post!
      So, the key to your question is your comment: “The solution you propose include a too many other addresses…” So, what rules do you use to decide if a summary has too many extra addresses? It all reduces to how you define what’s allowed in a summary route. That’s the key.

      I did supply my definition for best, back in the first post in the series, where I give some background on summary routes. In that post, I explain how we determine “best” in this context. Basically:

      … best summary generally means the summary route that includes all the addresses in the original subnets, but is the smallest such subnet.

      However, you are correct in that if you want to avoid including addresses inside the summary that are not in the originally listed subnets – a different definition of the “best’ summary – then you would indeed find a different result. For instance, your answer includes fewer addresses from outside the original subnets, but it includes some, so you have a different definition of “best summary”. I’m fine with that. But the exercise set-up was based on the definition I gave to start the series at http://www.ccentskills.com/summaryroute/.

      Hope this helps!
      Wendell

      Reply to this comment
      • adriana November 18, 20:01

        Hi Wendell,
        Thanks a lot for your prompt response … it has been very helpful.
        I have read your first post and I do understand your approach to summary routes.

        Would you mind to tell me if this is the approach used in CCNA certification exams?

        Adriana

        Reply to this comment
  2. CCENTSkills November 18, 20:56

    Sure thing, Adriana. You should be ready to look at the question, determine if the summary can or can’t include the other addresses or not, and the do the math. (I’m sure from your original question you can do the math.) It should be clear from the question or the context.

    Reply to this comment
  3. Robin December 10, 12:52

    Hi Wendell – i use the binary method to figure out summary addresses and have come across a discrepancy using this method with Q3 of set 1.
    If i use the binary values that are in common to work out the network address, this is 64 yet you give an answer of 72 for the network address.
    Is this because the prefix of the networks in your question are of varying lengths?

    Reply to this comment
    • CCENTSkills January 16, 11:27

      Hi Robin,
      Catching up on the blog… sorry for the delay.
      I’m not seeing where this post shows the answer – that is, the summary route – for problem 3 as ending in .72. I think it’s shown as .64. The table titled “Problem 3, Original Subnets…” has a .72. The final table, which shows the answer, shows the answer in the 2nd-to-last row as 192.168.1.64.
      If you’d like to follow up and tell me what I’m not seeing, that’s fine.
      Regards,
      Wendell

      Reply to this comment
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