Alittle confused about this question...

You are designing an IP addressing scheme for a company using classful addressing. You have been assigned a class C network address 192.168.24.0 and you have to divide it into 4 subnets with at minimum of 25 hosts each. What subnet should you use?
a. 255.255.0.0
b. 255.255.255.0
c. 255.255.255.192
d. 255.255.255.224
e. 255.255.255.240
f. 255.255.255.129

1 6 3 1 8 4 2 1
2 4 2 6
8

1 1 1 0 0 0 0 0

3 bits gives you up to 32 hosts / 25 is what they're looking for...

But what I don't get is how that gives you 4 subnets... because if you look at the following question...

28. You are designing an IP addressing scheme for a company using CIDR compliant routing protcol and routers configured with the ip subet-zero command. You have been assigned a class B network address 172.16.4.0 and you have to divide it into 15 subnets with at minimum of 3000 hosts each. What subnet should you use? a. 255.255.0.0
b. 255.255.192.0
c. 255.255.255.240
d. 255.255.224.0
e. 255.255.240.0
f. 255.255.248.0

1 6 3 1 8 4 2 1
2 4 2 6
8

1 1 1 1 0 0 0 0

16 gives you 15 subnets... So the answer is 255.255.240.0 ...of course this IS a class B address vs a class C address - but what I'm confused about is... why in the first question, each bit represents the amount of hosts, vs the second question where, each bit represents a subnet???

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Comments

mattau

the first question to get 4 subnets you would borrow 2 most left host bits from the 4th octet

before subnetting

00000000

after

11000000

2x2 is 4 subnets which leaves 2x2x2x2x2x2 -2 for valid hosts 62 hosts

255.255.255.192

it all depends on the question and what they want you to do. ie design a minimum of x subnets and only allow a maximum amount of hosts.

sometimes you might find it easier to work out the subnets first so you work from the left most bits or if you want to work out the hosts you work from the right most bits

so if they want 3000 hosts as a minimum and 15 subnets id probably just get the subnets out the way first then the rest can be hosts.

CompUBug

mattau wrote: »

the first question to get 4 subnets you would borrow 2 most left host bits from the 4th octet

before subnetting

00000000

after

11000000

2x2 is 4 subnets which leaves 2x2x2x2x2x2 -2 for valid hosts 62 hosts

255.255.255.192

it all depends on the question and what they want you to do. ie design a minimum of x subnets and only allow a maximum amount of hosts.

sometimes you might find it easier to work out the subnets first so you work from the left most bits or if you want to work out the hosts you work from the right most bits

so if they want 3000 hosts as a minimum and 15 subnets id probably just get the subnets out the way first then the rest can be hosts.

Well the problem is, according to the exam, the answer is 255.255.255.224

okplaya

This is likely a question from the old exams where it was assumed that ip subnet zero is NOT used. Nowadays, it's assumed that it IS used. You have to be careful with outdated study materials. Sure, the majority of the content will be relevant, but little things like these can be annoying.

HTH

thedrama

okplaya wrote: »

This is likely a question from the old exams where it was assumed that ip subnet zero is NOT used. Nowadays, it's assumed that it IS used. You have to be careful with outdated study materials. Sure, the majority of the content will be relevant, but little things like these can be annoying.

HTH

i consider the answer for this question might not even be C. Cos, even you
do not take ip-subnet zero into account, C will bring you 6 subnet not 4
and 30 hosts.

masterk

thedrama wrote: »

i consider the answer for this question might not even be C. Cos, even you
do not take ip-subnet zero into account, C will bring you 6 subnet not 4
and 30 hosts.

Surely though .192 is two subnet bits, 2 to the power of 2 is 4? So 4 subnets?

pizzaboy

Maybe they're looking for the subnet mask the would bring you closest to the number of hosts you require. When I did it in my head I would have gone with the option that would give me 30 hosts and more than the 4 possible subnets. IMO

masterk

pizzaboy wrote: »

Maybe they're looking for the subnet mask the would bring you closest to the number of hosts you require. When I did it in my head I would have gone with the option that would give me 30 hosts and more than the 4 possible subnets. IMO

Interesting way of looking at it, it does read to me though that as they state the "minimum" amount of required hosts is 25 and they just specify 4 subnets (no max or minimum), it seems they are pointing you to maxing out on hosts. Thats just from my experience of subnetting questions though.

chopsticks

CompUBug wrote: »

but what I'm confused about is... why in the first question, each bit represents the amount of hosts, vs the second question where, each bit represents a subnet???

The given answer for the first question is

"3 bits gives you up to 32 hosts / 25 is what they're looking for"

I think by re-phrasing the answer a little, it might help to clear your doubt

"3 (subnet) bits (generates a total of 8 subnets [6 subnets if ip zero subnet is used to reserve the first and last subnet], and each subnet) gives you up to 32 hosts / 25 is what they're looking for"