For all u subnetting guru's, can u answer this q's?????

MaddogZ

hi,
can some on help me i got this question from celticrover

Q. Which of the following subnet masks would allow a Class A network to have upto 150 hosts and upto 164 subnets?

a. 255.255.0.0
b. 255.255.255.0
c. 255.255.192.0
d. 255.255.240.0
e. 255.255.252.0
f. 255.255.255.192

Is it answer a and b? A because it allows for 8 bits of subnet and 16 bits of hosts and B because it allows for 16 bits of subnets and 8 bits of hosts?

In celticrover they say the answer is A 255.255.0.0

qsub

It can be A or B, it's just that A is the better answer.

Arturasj

None of the choices are correct.
255.255.255.0 Allows 254 hosts
255.255.0.0 Allows 65534 hosts
The question is asking which mask would allow “UPTO” 150 hosts what means equals or less than 150. None of the given masks can provide it. However 255.255.255.0 (B) is closer to 150 therefore I would say it would be a better option to meet a required condition.

qsub

You say it's 255.255.255, yeah, it's closer to the amount of hosts, but a lot furthur away from the amount of subnets.

It says upto doesn't it for both?

Either way, I say A is the better answer.

You want 164 subnets, using 255.255.0.0 will give you more obviously, but with a lot of hosts per subnet.

Easier to keep track of IMHO.

Arturasj

The reason I think that B is a better answer because when you’ve got two shots to hit the targets it is better to gave one close to the target and one far away than both almost equally far from the point.

Anyway, this is a pointless argument as there is no correct answer.

I don't think that such a question would come up in real exam, as cisco is trying to get very precise answers.

EricO

This is an exact replica of an end of chapter question in the ICND book. Think about it. Answers a, b, c, d, and e are correct because they all meet the criteria of the question being asked.

ubergeek

EricO wrote:

This is an exact replica of an end of chapter question in the ICND book. Think about it. Answers a, b, c, d, and e are correct because they all meet the criteria of the question being asked.

I agree with Eric, all of this 4 subnet masks will give you the hosts and subnets that would fulfill the requirements of the question.

Note:

255.255.0.0 would give you 256 subnets; 65534 usable IP addresses
255.255.255.0 -> 65536 subnets; 254 usable ip addresses
255.255.192.0 -> 1024 subnets; 16382 UIP
255.255.240.0 -> 4096 subnets; 4094 UIP
255.255.252.0 -> 16384 subnets; 1022 UIP

However if you used /26 it will only give you 262144 subnets though it will give you only 62 hosts per subnet.

Yankee

You are not reading the question as they intended. If you started with a /24 network (answer how many times could you split it to come up with the required 150 hosts? Look at it that way for each proposed mask and you will see A is the answer.

Yankee

ubergeek

lostlego wrote:

Q. Which of the following subnet masks would allow a Class A network to have up to 150 hosts and up to 164 subnets?

a. 255.255.0.0
b. 255.255.255.0
c. 255.255.192.0
d. 255.255.240.0
e. 255.255.252.0
f. 255.255.255.192

I regret to say that there was no entry on the question that it would start with a /24 and would splice to it to achieve the correct number of hosts and up to the desired number of subnets.. maybe my subnetting is getting worse

Yankee

Let me try and explain one last time

A says you have a /16
B says you have a /24
C says you have a /18
D says you have a /20
etc

Given you need 150 hosts you require a /24 assigned so which of the above options allows you 164 /24s and the answer is only A.

I don't spend much time explaining why something is right because I want the reader to think about my response. It is intentional. The smart ones get it and others show they don't.

Yankee

EricO

I have to respectfully disagree. The question did not say "Which of these answers most closely meets this criteria." It said, "Which of the following subnet masks would allow". Quite frankly, anwers a, b, c, d, and e would allow the specified number of hosts and subnets.

forbesl

Look at it like this as an example:

Note: It has to be assumed that you must use CIDR. Otherwise, 5 of the answers (a through e) would be correct. Since 5 answers cannot be chosen, CIDR is being used. In order to meet minimum hosts requirements you have to use a subnet mask with a prefix of /24; A /25 will give you only 126 hosts per subnet, and fewer hosts the higher the prefix. Now let's see which mask gives us both up to 164 subnets AND 150 hosts per subnet.

For the "Class A" network 55.1.0.0 how many subnets and hosts will you get with:


a) CIDR MASK 255.255.0.0

Subnets: 55.1.0.0/24 through 55.1.255.0/24 (meets and exceeds the 164 subnets, right?)
Hosts per subnet: .1 through .254 (more and exceeds the 150 host requirement, right?)

MEETS BOTH REQUIREMENTS

b) CIDR MASK 255.255.255.0

Subnets: 1 (not even close to 164 subnets)
Hosts per subnet: 254

DOESN'T MEET MINIMUM NUMBER OF SUBNETS REQUIRMENT

c) CIDR MASK 255.255.192.0

You can look at this and see it's not going to give you 164 /24 subnets
Hosts per subnet: 254

DOESN'T MEET MINIMUM NUMBER OF SUBNETS REQUIREMENT

d) CIDR MASK 255.255.240.0

You can look at this and see it's not going to give you 164 /24 subnets
Hosts per subnet: 254

DOESN'T MEET MINIMUM NUMBER OF SUBNETS REQUIREMENT

e) CIDR MASK 255.255.252.0

You can look at this and see it's not going to give you 164 /24 subnets
Hosts per subnet: 254

DOESN'T MEET MINIMUM NUMBER OF SUBNETS REQUIREMENT

f) CIDR MASK 255.255.255.192

WAY OUT OF THE BALLPARK!!!!

As you can tell, answer A would be the correct answer as Yankee stated previously.

EricO

Or.....


We could look at it like Cisco does in the ICND book....




and the answer:

forbesl

Yup,

It's the same question with one twist: Whereas Cisco has several correct answers, Celtic Rover has taken it one step further and used only one answer. Notice in the first post, he stated that Celtic Rover chose answer A (which would be correct assuming you're using CIDR). Assuming you're using classful routing (which Cisco appears to be using) the answers would indeed be b, c, d, e, f.

I guess it all depends on who is asking the question. On an actual Cisco certification exam, you would know how many answers you are suppose to choose. If you have to choose only one answer, you get a "radio button" for the choices. If you have to choose multiple answers, you get "check boxes" and Cisco even states "choose X" where "X" is the number of correct answers.