subnetting question

The question asks me how many host and subnets will I gain if I go from 172.17.32.0/23 to a 172.17.32.0/27

Answers:

A. 8 subnets and 31 hosts
B. 8 subnets and 32 hosts
c. 16 subnets and 30 hosts

It's saying the answer is C, but the answer be B? Any thoughts?

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Comments

capitanuionut

mallyg27 wrote: »

The question asks me how many host and subnets will I gain if I go from 172.17.32.0/23 to a 172.17.32.0/27

Answers:

A. 8 subnets and 31 hosts
B. 8 subnets and 32 hosts
c. 16 subnets and 30 hosts

It's saying the answer is C, but the answer be B? Any thoughts?

The correct answer is C ...

172.17.32.0/23 - is from 172.17.32.0 to 172.17.33.255 - 512 addresses

So if you split this into 32 address subnets it gives you a total of 16 subnets ... minus 1 broadcast address and 1 network address it gives you 16 subnets with 30 host addresses each

of simple ... you borrow four more bits from the host portion ...27 - 23 = 4...2 ^ 4 = 16 subnets

if remains 5 bits for hosts .. 2^5 - 2 = 30 hosts for subnet

Chris_

Unless I'm missin something in the question, the answer is C - what makes you think it's B

you've got 9 bits to play with from the /23, 4 are now used for network leaving 5 for hosts.

2^4 = 16 for the subnets

2^5 = 32 for the hosts, -2 for network and B'cast = 30.

mallyg27

I got it. Just reading the question wrong.Thanks guys.

mallyg27

My question is, won't we be losing host though if we go from the /23 to /27?

Chris_

Effectively, yes - with the /27 subnets you've got 480 available hosts across them all as opposed to 510 with the /23 - so I suppose the use of the word 'gain' in the question only applies to the subnets - get used to badly worded questions, you'll see a lot of them in the cisco Certs world.

mallyg27

Chris_ wrote: »

Effectively, yes - with the /27 subnets you've got 480 available hosts across them all as opposed to 510 with the /23 - so I suppose the use of the word 'gain' in the question only applies to the subnets - get used to badly worded questions, you'll see a lot of them in the cisco Certs world.

Now im confused now. Wouldnt a /27 give you 2^5= 30 host and /23 give you 2^9 =510 host?

JohnnyBiggles

mallyg27 wrote: »

Now im confused now. Wouldnt a /27 give you 2^5= 30 host and /23 give you 2^9 =510 host?

+1

I agree....

172.17.32.0 = Class B which is 255.255.0.0 = 11111111.11111111.00000000.00000000

/23 = 11111111.11111111.11111110.00000000
a) 2^7 = 128 = # of subnets
b) increment is 1 on the 3rd octet
c) (2^9)-2 = 512-2 = 510 hosts

/27 = 11111111.11111111.11111111.11100000
a) 2^11 = 2048 = # of subnets
b) increment is 32 which is also the block size on the 4th octet
c) (2^5)-2 = 32-2 = 30 hosts

SO, ?????????????????????????????????????? how are any of those correct? How do you gain subnets AND hosts going from /23 to /27 ??

Am I way off here??

mallyg27

JohnnyBiggles wrote: »

+1

I agree....

172.17.32.0 = Class B which is 255.255.0.0 = 11111111.11111111.00000000.00000000

/23 = 11111111.11111111.11111110.00000000
a) 2^7 = 128 = # of subnets
b) increment is 1 on the 3rd octet
c) (2^9)-2 = 512-2 = 510 hosts

/27 = 11111111.11111111.11111111.11100000
a) 2^11 = 2048 = # of subnets
b) increment is 32 which is also the block size on the 4th octet
c) (2^5)-2 = 32-2 = 30 hosts

SO, ?????????????????????????????????????? how are any of those correct? How do you gain subnets AND hosts going from /23 to /27 ??

Am I way off here??

Actually the increment is two for the /23

JohnnyBiggles

mallyg27 wrote: »

Actually the increment is two for the /23

Typo... but the questions still apply.

Chris_

JohnnyBiggles wrote: »

+1

I agree....

172.17.32.0 = Class B which is 255.255.0.0 = 11111111.11111111.00000000.00000000

/23 = 11111111.11111111.11111110.00000000
a) 2^7 = 128 = # of subnets
b) increment is 1 on the 3rd octet
c) (2^9)-2 = 512-2 = 510 hosts

/27 = 11111111.11111111.11111111.11100000
a) 2^11 = 2048 = # of subnets
b) increment is 32 which is also the block size on the 4th octet
c) (2^5)-2 = 32-2 = 30 hosts

SO, ?????????????????????????????????????? how are any of those correct? How do you gain subnets AND hosts going from /23 to /27 ??

Am I way off here??

I was just saying that the /23 mask gives you 510 useable hosts on the 1 subnet and the /27 gives you 480 hosts total across ALL the subnets you've created (16 subnets * 30 useable hosts = 480 hosts)

I said this to point out that the question was a stupid one -as you don't gain subnets and hosts - you just gain subnets.

Chris_

mallyg27 wrote: »

My question is, won't we be losing host though if we go from the /23 to /27?

I was agreeing with this statement !!!!!!!!

mallyg27

Chris_ wrote: »

I was just saying that the /23 mask gives you 510 useable hosts on the 1 subnet and the /27 gives you 480 hosts total across ALL the subnets you've created (16 subnets * 30 useable hosts = 480 hosts)

I said this to point out that the question was a stupid one -as you don't gain subnets and hosts - you just gain subnets.

Where are you getting 16 subnets from? Doesn't a /27 give us 2^11=2048 subnets?

Chris_

mallyg27 wrote: »

Where are you getting 16 subnets from? Doesn't a /27 give us 2^11=2048 subnets?

er - it does if you're starting off with a class B /16 address and then subnetting it down to a /27

That's not what you asked - you're starting off with a /23, already subnetted address. If you've been given 172.17.32.0/23 (effectively the range 172.17.32.0 - 172.17.33.255) and asked to subnet it into /27 subnets, that gives you 4 bits to play with for your subnets = 16

You can't just use the rest of that /16 - for example 172.17.34.0/23 may hav been given to another admin for his site.

JohnnyBiggles

Chris_ wrote: »

er - it does if you're starting off with a class B /16 address and then subnetting it down to a /27

That's not what you asked - you're starting off with a /23, already subnetted address. If you've been given 172.17.32.0/23 (effectively the range 172.17.32.0 - 172.17.33.255) and asked to subnet it into /27 subnets, that gives you 4 bits to play with for your subnets = 16

You can't just use the rest of that /16 - for example 172.17.34.0/23 may hav been given to another admin for his site.

I think I get it now.... but that indeed is a horrible question... or at least horribly worded.. or just a bad concept. Subnet a subnetted address? How common is that??

SO, in a nutshell:

Subnet this network address... (which is already subnetted since the mask is not the default Class B mask)
172.17.34.0/23
using this mask...
255.255.255.224 (/27)

Here goes:

172.17.34.0/23
/23 = 11111111.11111111.11111110.00000000
/27 = 11111111.11111111.11111111.11100000
a) 2^4 = 16 = # of subnets gained
b) increment is 32 on the last octet
c) (2^5)-2 = 32-2 = 30 hosts per subnet gained

EDIT: My question now is, are you actually gaining subnets and hosts or is the new # of subnets and new # of hosts just a split from the total from the /23 network? In other words, if you had 2 stacks of 100 1-dollar-bills (=$200 total), and were told to break those stacks up into 8 smaller stacks... you'd have 8 stacks of 25 1-dollar-bills (still =$200)

Same concept? I didn't do the math for the subnetting beyond what I put above...lol

EDIT 2:

Chris_ wrote: »

I said this to point out that the question was a stupid one -as you don't gain subnets and hosts - you just gain subnets.

OOPS! You already answered my question. Thanks.