Overlapping VLSM Subnets – Speed Test 3 Answers

As usual, the “answers” post only makes sense after reading the “question” post. Use the question as a place to practice and ignore the speed and time pressure, or use it truly as a speed test. Either way, your job: any way you know how, derive the subnet IDs, and the range of addresses in each subnet, and then compare the subnets to identify which of the subnets have a range of addresses that overlaps with other subnets in the list. That’s it!

If you’re not sure what to do, look at the following post first, which puts it in perspective, and then come back here for the problem and the speed challenge. Here’s a complete list of related posts:

• Intro to overlapped subnet problems
• All Overlapped Subnet problems
• IPv4 Subnet Speed Tests

Answer

Items 1 and 5 overlapped

You can use any process you want, but here’s what I do. If you have questions on any individual calculation, ask away. Once you know the subnet ID and subnet broadcast address for each, just look for the overlaps.

1) Find the subnet IDs (if starting with IP address/prefix combinations)

2) List from lowest subnet ID to highest

3) List the broadcast address for each subnet next to the subnet ID

4) Compare adjacent entries to look for overlaps

5) If you find an overlap, when comparing the next item, compare to all subnets already known to overlap

Once sorted, you can find the first pair of overlaps working through the list sequentially. Here’s the sorted list.