This post lists the answers to summary route drill 3. If you’ve not tried the problems for this drill yet, go back, check it out, and come up with the best summary routes. Then come back here and check your answers. Here are some other useful links as well:

(You can ignore this section if you’ve used the other Summary Route Drills before.)

The answer to each of the three problems uses two tables. The first table lists the original masks and subnet IDs. It also lists the subnet broadcast addresses. The subnet ID and subnet broadcast address for each subnet brackets the low and high end of the numbers in that subnet, respectively.

The best summary route will be the first subnet/mask combination found by the process that includes both the low and high-end numbers in this table from all the subnets.

The second table for each answer lists the calculation of potential correct answers per my suggested process. The table shows each successive calculation of a new subnet ID, using a new /P mask, and its broadcast address. Each row of the table shows the results of a new calculation, just with each /P being 1 smaller than the previous value.

Note that the second table lists an extra correct but suboptimal answer. The last row in the 2nd table for each problem includes a final summary route with a mask (/P) one shorter than the correct best summary route. This last route could be used as a summary, but is not considered the best summary route, because it includes an unnecessarily broad range of addresses.

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.

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. The correct answer is in bold, in the 2nd to last table entry.

Problem 1, Table 2: Calculation Using 10.7.32.0, New /P

 Mask Calculated Subnet Broadcast Address /19 10.7.32.0 10.7.63.255 /18 10.7.0.0 10.7.63.255 /17 10.7.0.0 10.7.127.255 /16 10.7.0.0 10.7.255.255

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.

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. The correct answer is in bold, in the 2nd to last table entry.

Problem 2, Table 2: Calculation Using 172.20.180.0, New /P

 Mask Calculated Subnet Broadcast Address /20 172.20.176.0 172.20.191.255 /19 172.20.160.0 172.20.191.255 /18 172.20.128.0 172.20.191.255 /17 172.20.128.0 172.20.255.255 /16 172.20.0.0 172.20.255.255

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.

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. The correct answer is in bold, in the 2nd to last table entry.

Problem 3, Table 2: Calculation Using 192.168.8.100, New /P

 Mask Calculated Subnet Broadcast Address /29 192.168.8.96 192.168.8.103 /28 192.168.8.96 192.168.8.111 /27 192.168.8.96 192.168.8.127 /26 192.168.8.64 192.168.8.127
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