Practice for Chapter 6: Route Summarization

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2 A P P E N D I X E Practice for Chapter 6: Route Summarization This appendix includes two features. First, it contains extra practice problems and answers related to Chapter 6, "Route Summarization," specifically about how to summarize routes. The second feature shows how to summarize routes using binary, as promised in Chapter 6, in case you want to understand what happens in binary when you summarize routes. Note that you can find additional practice at the author s CCNA blog, which is linked from the author s website. Practice Finding the Best Summary Route Table E-1 lists four sets of subnets that need to be summarized as part of a summary route. Find the subnet number/mask combination that is the best summary route. Table E-1 Practice Problems: Finding the Best Summary Route Problem 1 Problem 2 Problem 3 Problem / / / / / / / / / / / / / / / /30 Answers to Practice Problems This section lists the answers to the practice problems along with a description of how to use the process in this book to solve the problems. For each problem, the first table lists the results of the first two steps; the grayed boxes show the low and high end of the range that the new summary route must enclose. The second table for each problem shows the results of each pass through Step 4, with the final (right-most) pass showing the correct answer.

3 3 Appendix E: Practice for Chapter 6: Route Summarization Practice Problem 1 Table E-2 Practice Problem 1: First Two Steps Subnet IDs/Masks Subnet Broadcasts For Problem 1, at Step 3, the shortest mask is /21, so the initial mask will be one smaller, or /20. Finding the correct answer requires four passes through calculating a new subnet ID and mask, with the final answer shown in Table E-3. Table E-3 Practice Problem 1: Multiple Passes Through Step 4 (Correct Answer Highlighted) All Passes Use st Pass: /20 2 nd Pass: /19 3 rd Pass: /18 4 th Pass: /17 Subnet ID Broadcast Address The best summary route will use subnet ID , mask /17. Practice Problem 2 Table E-4 Practice Problem 2: First Two Steps Subnet IDs/Masks Subnet Broadcasts

4 Practice Finding the Best Summary Route 4 For Problem 2, at Step 3, all the original masks are /23, so the initial mask will be one smaller, or /22. Finding the correct answer requires three passes through calculating a new subnet ID and mask, with the final answer shown in Table E-5. Table E-5 Practice Problem 2: Multiple Passes Through Step 4 (Correct Answer Highlighted) All Passes Use st Pass: /22 2 nd Pass: /21 3 rd Pass: /20 Subnet ID Broadcast Address The best summary route will use subnet ID , mask /20. Practice Problem 3 Table E-6 Practice Problem 3: First Two Steps Subnet IDs/Masks Subnet Broadcasts For Problem 3, at Step 3, all the original masks are /25, so the initial mask will be one smaller, or /24. Finding the correct answer requires four passes through calculating a new subnet ID and mask, with the final answer shown in Table E-7. Table E-7 Practice Problem 3: Multiple Passes Through Step 4 (Correct Answer Highlighted) All Passes Use st Pass: /24 2 nd Pass: /23 3 rd Pass: /22 4 th Pass: /21 Subnet ID Broadcast Address The best summary route will use subnet ID , mask /21.

5 5 Appendix E: Practice for Chapter 6: Route Summarization Practice Problem 4 Table E-8 Practice Problem 4: First Two Steps Subnet IDs/Masks Subnet Broadcasts For Problem 4, at Step 3, the shortest mask is /28, so the initial mask will be one smaller, or /27. Finding the correct answer requires only two passes through calculating a new subnet ID and mask, with the final answer shown in Table E-9. Table E-9 Practice Problem 4: Multiple Passes Through Step 4 (Correct Answer Highlighted) All Passes Use st Pass: /27 2 nd Pass: /26 Subnet ID Broadcast Address The best summary route will use subnet ID , mask /26. Reference: A Binary Process to Find the Best Summary Route This section lists a binary process to find the best summary route for a given set of subnets, along with an example. You may choose to completely ignore this section, or use it if you are interested. The following list describes a generalized binary process by which you can find a best summary route for a group of subnets: Step 1 Step 2 Step 3 List all to-be-summarized subnet numbers in binary. Find the first N bits of the subnet numbers for which every subnet has the same value, moving from left to right. (For our purposes, consider this first part the in-common part.) To find the summary route s subnet number, write down the in-common bits from Step 2, and binary 0s for the remaining bits. Convert back to decimal, 8 bits at a time, when finished.

6 Reference: A Binary Process to Find the Best Summary Route 6 Step 4 Step 5 To find the summary route s subnet mask, write down N binary 1s, with N being the number of in-common bits found at Step 2. Complete the subnet mask with all binary 0s. Convert back to decimal, 8 bits at a time, when finished. Check your work by calculating the range of valid IP addresses implied by the new summary route, comparing the range to the summarized subnets. The new summary should encompass all IP addresses in the summarized subnets. By looking at the subnet numbers in binary, you can easily discover the bits in common among all the subnet numbers. By using the longest number of bits in common, you can find the best summary. The next two sections show two examples using this process to find the best, most concise, tightest summary routes for the network shown in Figure 5-1. Sample Best Summary on Seville Seville has subnets , , , and , all with mask You start the process by writing down all the subnet numbers in binary: Step 2 requires that you find all in-common bits at the beginning of all the subnets. Even before looking at the numbers in binary, you can guess that the first two octets are identical in all four subnets. So, a quick look at the first 16 bits of all four subnet numbers confirms that all have the same value. This means that the in-common part (Step 2) is at least 16 bits long. Further examination shows that the first 6 bits of the third octet are also identical, but the seventh bit in the third octet has some different values among the different subnets. So, the in-common part of these four subnets is the first 22 bits. Step 3 says to create a subnet number for the summary by taking the same bits in the incommon part, and write down binary 0s for the rest. In this case: Step 4 creates the mask by using binary 1s for the same bits as the in-common part, which is the first 22 bits in this case, and then binary 0s for the remaining bits, as follows:

7 7 Appendix E: Practice for Chapter 6: Route Summarization So, the summary route uses subnet , mask Step 5 suggests a method to check your work. The summary route should include all the IP addresses in the summarized routes. In this case, the range of addresses for the summary route starts with The first valid IP address is , the final valid IP address is , and the broadcast address is In this case, the summary route includes all the IP addresses in the four routes it summarizes and no extraneous IP addresses. Sample Best Summary on Yosemite The four subnets on Yosemite cannot be summarized quite as efficiently as those on Seville. On Seville, the summary route itself covers the same set of IP addresses as the four subnets with no extra addresses. As you will see, the best summary route at Yosemite includes twice as many addresses in the summary as exist in the original four subnets. Yosemite has subnets , , , and , all with mask The process starts at Step 1 by writing down all the subnet numbers in binary: At Step 2, it appears that the first two octets are identical in all four subnets, plus the first 5 bits of the third octet. So, the first 21 bits of the four subnet numbers are in common. Step 3 says to create a subnet number for the summary route by taking the same value for the in-common part and binary 0s for the rest. In this case: Step 4 creates the mask used for the summary route by using binary 1s for the in-common part and binary 0s for the rest. The in-common part in this example is the first 21 bits: So, the best summary is , mask Step 5 suggests a method to check your work. The summary route should define a superset of the IP addresses in the summarized routes. In this case, the range of addresses starts with The first valid IP address is , the final valid IP address is , and the broadcast address is In this case, the summary route summarizes a larger set of addresses than just the four subnets, but it does include all addresses in all four subnets.

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