Interview Question

I had a phone screen interview yesterday which I managed to pass but I had one question tha really stumped me. It may have just been the wording that threw me but here's the question:

How many /27 networks can you fit into a /24?

Now that I think more about it I'm not sure if they meant with the bits used which would give us
8 networks since 3 bits are used for subnetting or of there's more to it. Any thoughts?

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Comments

malcybood

Yeah I think they were checking if you knew about summarization....I would have said 8 subnets but 6 usable if I was going by the book.

The wording was not great though of their question

dynamik

bestialwarlust wrote:

8 networks since 3 bits are used for subnetting or of there's more to it.

That's what I would have gone with. I think you're over-thinking it.

bestialwarlust

Yeah thats what I figured. They were throwing alot of questions at me so that was kind of a curve ball.

itdaddy

man this may sound dumb but where can i get this reference is this subject taught in CCNP courses..

stupid questiion but you can actually put a /27 into a /24 isnt it tooo big?
i thought a /24 will fit in a /27 but not the reverse???

shednik

itdaddy wrote:

man this may sound dumb but where can i get this reference is this subject taught in CCNP courses..

stupid questiion but you can actually put a /27 into a /24 isnt it tooo big?
i thought a /24 will fit in a /27 but not the reverse???

This was taught on the CCNA...the /27 makes each subnet have a block size of 32

so say you had the network 192.168.1.0/24 and wanted to split that up into a /27

192.168.1.0
192.168.1.32
192.168.1.64
192.168.1.96
192.168.1.128
192.168.1.160
192.168.1.192
192.168.1.224

Now the first and last are not use-able if ip subnet zero in not enabled, but it is by default so I would have said 8.

Plazma

Yea.. that's definatly a tricky one.. I had to think about it for a second myself.. at first glance i was like "WTF?"

bighornsheep

itdaddy wrote:

man this may sound dumb but where can i get this reference is this subject taught in CCNP courses..

stupid questiion but you can actually put a /27 into a /24 isnt it tooo big?
i thought a /24 will fit in a /27 but not the reverse???

shednik wrote:

192.168.1.0
192.168.1.32
192.168.1.64
192.168.1.96
192.168.1.128
192.168.1.160
192.168.1.192
192.168.1.224

Now the first and last are not use-able if ip subnet zero in not enabled, but it is by default so I would have said 8.

ditto.....

bestialwarlust wrote:

Yeah thats what I figured. They were throwing alot of questions at me so that was kind of a curve ball.

Anything about IP headers....and bit lenths? Those are truly wtf.......

btw, what's the gig? good luck dude...don't forget to send thank you and followup!

bestialwarlust

bighornsheep wrote:

itdaddy wrote:

man this may sound dumb but where can i get this reference is this subject taught in CCNP courses..

stupid questiion but you can actually put a /27 into a /24 isnt it tooo big?
i thought a /24 will fit in a /27 but not the reverse???

shednik wrote:

192.168.1.0
192.168.1.32
192.168.1.64
192.168.1.96
192.168.1.128
192.168.1.160
192.168.1.192
192.168.1.224

Now the first and last are not use-able if ip subnet zero in not enabled, but it is by default so I would have said 8.

ditto.....

bestialwarlust wrote:

Yeah thats what I figured. They were throwing alot of questions at me so that was kind of a curve ball.

Anything about IP headers....and bit lenths? Those are truly wtf.......

btw, what's the gig? good luck dude...don't forget to send thank you and followup!

Network security admin. I dont' have alot of security in my background but if I can get my foot in the door this will be a great opportunity. Nothing about IP headers some IP sec questions I wasn't sure on. But apparently I have enough of a solid basic networking routing, switching where I was able to swing the face to face interview. I'm out of a job and need this one real bad. Been doing onctracting working but the work mostly been desktop. All my CCNA skills are getting dusty and rusty so I hope this one works out.

itdaddy

bighornsheep

hahha

I know what he means I was just seeing if you guys could explain it.

i know that /24 is a class c and then you add the /27 on the end using bit notation.
just wanted to get some responses from you guys and yes i did.

what I should have asked was how do you subnett hahaha hahah haha
then I could get some real comments....just wanted an explain from yo guys
that is all I know what he means...ahah did you see the look on your avators face ahaha haha
hahah

now wasnt that fun to explain it for our refresh course dudesyou rock!

what i do is this /27 - /24 is 2^3 = 8 subnets ornetworks

so if ip-subnet zero was enabled I do believe then it is 8 if not then it is only 6. you guys are right

itdaddy

Anything about IP headers....and bit lenths? Those are truly wtf.......

btw, what's the gig? good luck dude...don't forget to send thank you and followup!

love it when they ask about that TCP and UDP headers...like we sit around and look at that all the time....looking for bobby fisher I say!

question, what is an IP header or TCP header??? ahahha ahahhaa hhaha ha
hahahh

itdaddy

Yea.. that's definitely a tricky one.. I had to think about it for a second myself.. at first glance i was like "WTF?"

plazma you kill me yeah, i dont like the way it is said but arse backwords but yeah
router summary is easy once you know how to use the bit notation and find the last bit
or the bit positon that the /27 is in or all match and then making your subnet mask..
oooooh i love review......

kryolla

What malcybood said

summarization and not subnetting a /24 to more usable networks /27, you always summarize to a smaller mask and subnet to a larger mask. What is the summary address for the below networks

192.168.1.0 /27
192.168.1.32 /27
192.168.1.64 /27
192.168.1.96 /27
192.168.1.128 /27
192.168.1.160 /27
192.168.1.192 /27
192.168.1.224 /27

It would be 192.168.1.0 /24

itdaddy

summarization and not subnetting a /24 to more usable networks /27, you always summarize to a smaller mask and subnet to a larger mask. What is the summary address for the below networks

192.168.1.0 /27
192.168.1.32 /27
192.168.1.64 /27
192.168.1.96 /27
192.168.1.128 /27
192.168.1.160 /27
192.168.1.192 /27
192.168.1.224 /27

It would be 192.168.1.0 /24

kryolla

you said it better summarize to smaller from question weird how many /27 in a /24.
is that the right kind of questions? can you rephrase the question better.?

kryolla

when you summarize you are taking a bunch of network that are contigous into 1 summary address. In order to do that you have to find all the bits that they all have in common which will make the subnest mask smaller.