## 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:

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.

 Order in Original Question Subnet ID Subnet Broadcast 3 172.16.8.0 172.16.11.255 5 172.16.12.0 172.16.15.255 1 172.16.14.0 172.16.15.255 2 172.16.17.128 172.16.17.255 4 172.16.20.240 172.16.20.243

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Wendell are you saying overlapping variable length subnets will always have the same Subnet Broadcast address?

It seems the end goal of your example is to produce a list of Subnet Broadcast addresses to be reviewed for duplicate entries. (Am I understanding your method correctly)?

I am currently preparing for CCNA exam using your study guides and they have been extremely beneficial in boosting my confidence for the tests AND my skills as a network admin, but am challenged by VLSM overlap topic.

Thanks for any info or assistance you can provide.
Doug A.

Hi Doug,
I don’t think I generalized the example into a rule of “overlapping VLSM subnets always have the same subnet broadcast address”. They may, or may not.

For instance, if this example had used 172.16.13.1/23 for subnet 1 (instead of 172.16.15.1/23), then #1 and #5 would still have overlapped. However, the subnet broadcast addresses would not have been the same – 172.16.13.255 and 172.16.15.255, respectively.

Hope this helps! Sorry for the delay – looks like my blog comment notices were ending up in Spam.
Wendell

Thanks for the depicted process! It’s really helpful.

But I wonder what algorithm do cisco equipments use to determine the typed subnets are overlapped or not?

Sure thing, Amil.
As far as the algorithm Cisco wrote in say IOS, it’s not knowable outside of looking at the code. It’s the same logic as “is this number in that subnet”, which of course routers already do habitually for routing. I imagine it’s the same code. EG, to detect an overlap of two subnets, take subnet ID one, subnet ID/mask 2, and run that same algo to find “is subnet ID 1 in subnet ID/mask 2? Then do the reverse. If either are true, it’s an overlap. So adding the “is there an overlap” part is probably pretty simple. At least that’s what I would have done. 🙂
Wendell