## Overlapping VLSM Subnets – Speed Test 3

It’s time for the last #CCENT VLSM speed test for a while. 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! Sounds simple, but it really shows how comfortable you are with subnetting math.

Before you do your first VLSM Subnet Speed Test, look at the following post first, which puts these questions in perspective. Here’s a complete list of related posts:

## Tasks: Find the Range for Each Subnet; Then Find the Overlaps

The list shows IP addresses and masks. Derive the subnet IDs, and then identify which subnets overlap with each other. That is, if the subnets listed as #1 and #2 in the list overlap, list “1&2” as your answer.

1. 172.16.15.1/23
2. 172.16.17.131/25
3. 172.16.9.131/22
4. 172.16.20.241/30
5. 172.16.12.1/22
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[…] 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 […]

1&2 Are overlaping

I had 1 and 5 as overlapping

172.16.15.1/23 Range: 172.16.14.0 – 172.16.15.255

172.16.12.1/22 Range: 172.16.12.0 – 172.16.15.255

Hi John,
Thanks for the post! note that the answers post is linked at the bottom of the page as well, so you can check your answer. I won’t specifically reply with an “agree/disagree” here, to avoid spoiling the answer for others. Thanks!
Wendell

it took me 177 secs.does it okay?

I’d say you might work on speed a little. That tells me your probably at 25-30 seconds each subnet to find the range of addresses – maybe work to get that down to 20 or so.

Took me 96 seconds. So ~20s to find each range. I think I will struggle to get this overall time down as this requires you to write stuff down then review at the end when you are looking for overlaps. Is that time ok?

Hi Bav,
Yep, I’d say you’re in good shape. When you get to that small a number of seconds, the time to write it down does impact the total enough to matter, but I’m thinking more in terms of how much time for the answer to pop into your head. EG, if you quickly (1-2 seconds) realize 3 of the octets values, and then take another 12-13 seconds to calculate the interesting octet, then you’re in good shape.
W

how is it possible answer this in 20 seconds?

Hi Heks,
20 seconds for the whole problem is truly fast. You don’t need to set that as a goal. However, if you practice the underlying process (find the range of addresses in a subnet) enough, this one can be solved in under 20 seconds. For instance, start with the shorter masks (/22’s in this case), think of their ranges, and then look. Having not looked at this one in a while, I thought through the items and answers in about 12 seconds. But I admittedly practice the math a lot as part of my job. 20 seconds per subnet is pretty reasonable.