Hypothetical Interview Question - Subnetting

Hey everyone, brand new member and this is my first post. I apologize in advance if there was an introduction section in the forums that I overlooked. I saw a recent thread regarding the importance of subnetting knowledge.

I would imagine that in an interview a subnetting question would come up and I have a question on how to handle it. Let's say the position is for a junior to mid level network engineer position which required a CCNA.

My question is: would the average interviewer expect candidates to be able to subnet in their head? For example, if the question were, what subnet mask would grant X number of networks and Y number of hosts, would it look badly on me if I had to work it out on paper, write the subnet in binary and work it out from there? Thoughts are greatly appreciated and I look forward to interacting more on this forum in the future.

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Comments

Node Man

I would venture that they want you to do it in your head, because they have it in their head. Another way to look at it, is that you should not have to do the math, because you already have the chart memorized.

If anything, i would be honest about my skill with subnetting during an interview.

apr911

If you were applying for a networking position, I would expect you to be able to do most of it in your head. I certainly wouldnt mind if I asked you to give me the number of hosts in a /13 (255.248.0.0) network if you needed to "****" a bit with a piece of paper and do something like 256x256x8 (-2 for hosts) on a piece of paper as many people arent good with large numbers, especially doing math with large numbers, but I certainly wouldnt be expecting you to break out in binary. I can give you the rough number of approx. 524k hosts doing the math in my head quickly but to get the actual 524286 hosts (524288 IPs) its quicker for me to write some of it out (fortunately, Im gifted in number manipulation in my head so I could work it out entirely in my head, it would just take considerably longer than if I were to write out some portion).

When I was first getting into IT fulltime as a career, I applied for a server admin position and was asked a subnetting question during the interview. Not having the experience I do now, I broke it out into binary. I dont know if they were impressed or secretly laughing at my response but it was definitely a memorable experience for them because when I took the job 2 months later one of the interviewer recognized me as the guy who did subnetting in binary (they go through a lot of interviews at this place because of their size so to even be recognized is a stretch, to be remembered for a specific event in the interview... well thats something. I dont know what [good or bad] but its something.)

Ultimately though, accuracy is more important than methodology. The guys who interviewed me way back when said that I was the only one to break it down into binary but I was also one of a very small few to get the question right.

Even in a Networking position, I would rather someone who can do the math manually in binary and get the answer correct than someone who can do the math in their head and get the answer wrong.

Slow/Correct = less work getting done, Fast/wrong = more work being added. Slow/Correct is ok, Fast/wrong is bad. Id rather someone who is slowly going to contribute to the work that needs doing than someone who is quickly going to add work to the pile.

Still though, we all strive to be both fast and correct and if you're breaking out the binary for subnetting, you need to look at how you do subnetting as there are dozens of shortcuts that dont involve rote memorization.

Ive been doing networking now for several years and I still havent bothered to memorize most subnet masks (/8,/16,/24 are the only ones I actively memorized as they are what I refer to as "boundary" subnets where a mask smaller impacts the IPs in the masks octet and a mask larger impacts the IPs in the next octet). Most other subnets I calculate on the fly using my fingers to count powers of 2 and my head to calculate the powers of 2 and do the remaining math. There's nothing really stopping me from doing it all in my head but Ive found when I use my fingers as an aide when counting powers of 2, I reach the answer much more quickly and with much less error so its what I do.

Deathmage

apr911 wrote: »

There's nothing really stopping me from doing it all in my head but Ive found when I use my fingers as an aide when counting powers of 2, I reach the answer much more quickly and with much less error so its what I do.

I've heard of this finger subnetting, how did you learn?

kohr-ah

If I interviewed you I wouldn't hold it against you. As long as you can figure it out I'd all that would matter to me. I don't care if you need paper.

Of course I wouldn't ask you CCNA style like that either. I would just ask you can you tell me what the mask is of a /26.

wtrwlkr

Thanks for the replies all. The method I'm using to learn subnetting is the one taught by Jeremy from CBT Nuggets. He has you write the default subnet mask in binary, find the host increment and reserve host/network bits. I tried using **** charts and the like but I felt like I was using rote memorization rather than getting the "why" behind it. Jeremy's method helps me understand the material. In studying for my CCENT/CCNA, I plan on devoting at least 30 minutes a day practicing different types of subnetting questions. I assume (and hope) that with enough practice I'll be able to do most of it in my head.

apr911

Deathmage wrote: »

I've heard of this finger subnetting, how did you learn?

Its not really finger subnetting so much as using my finger to keep track of where I am.

For example, if Im told we're looking for a /19 network mask, I know the next boundary point is /24 which is 5 powers of 2 greater than /19 (24-19=5).

Instead of trying to track how many powers of 2 I need, how many powers of 2 I have and what that power of 2 is equal to all in my head, I offload the number of powers of 2 I have to my fingers:

I.e.
I use fingers for the 1st column (instead of in my head) and count in my head the second column up to the 5th power:
1 - 2
2 - 4
3 - 8
4 - 16
5 - 32

256-32 = 224
/24 = 255.255.255.0
/19 = 255.255.224.0

Ive gotten closer to memorizing common powers of 2 but in a pinch or if I want to be really certain of accuracy or you need me to give you a CIDR notation from a mask (I can memorize 2^5=32 but for some reason I haven't been able to memorize that 32=2^5), I still break it out on my fingers. This is a bit of a holdover from my server days... I can very quickly count in my head up to or back from 1048576 or 2^20 which is the equivalent of 1TB to MB or 1GB to KB, or 1MB to B, etc.

While I can tell you 1048576 is a power of 2 if you asked me which power of 2, it would take me considerably longer to count both the power and the number in my head than to count to the number while tracking the power with my fingers.

Im sure I could go higher but it definitely gets slower after that. Additionally, the math involved to calculate Harddrive size vs available space is more complicated than I can do in my head so usually I use it for things that use the 2 powers (i.e. memory sticks) and Ive yet to run into a server needing more than 2TB of memory (2^21 MB).

This is by far the best write up Ive seen on subnetting and how to "****" the system. Its still the system I use to this day as its able to go back and forth between CIDR notation and subnet mask with ease.

http://www.techexams.net/forums/ccna-ccent/38772-subnetting-made-easy.html

goldenlight

I would ask for a piece of paper and pencil unless it was something obvious.

Mutata

I have never been asked a subnetting question on an interview.

I find this rather strange.. lol

kurosaki00

the only thing Ive had asked to me is masks.. never got a subnet this for me question in an interview

AwesomeGarrett

I've only been asked for subnet mask during interviews. Asking for the number of host past a /22 is absurd, in my opinion. What does it prove?