What do you think the correct answer is???

viper75

Can some one explain this to me. Don't understand why I got this question wrong on a practice exam. (PrepLogic)

Before I give the answer they marked as correct I want to see what everyone else will select to be the answer.

This is a question from the PrepLogic CCNA Practice exam word for word.

Your organization is using the 198.133.219.0/24 address space for it's Internet presence systems. The IP address plan provides for 4 subnets within the given address space. What network mask are these host in your organization using?

A) 255.255.255.4

255.255.255.8

C) 255.255.255.192

D) 255.255.255.0

E) 255.255.255.224

garv221

I'm guessing you Guessed answer "D"?

viper75

garv221 wrote:

I'm guessing you Guessed answer "D"?

No, I picked something else.

garv221

C?

viper75

garv221 wrote:

C?

Well What I'm looking for is to see why someone picked the answer they did. The logic behind it.

garv221

viper75 wrote:

garv221 wrote:

C?

Well What I'm looking for is to see why someone picked the answer they did. The logic behind it.

I would say C b/c I have 256 addresses w/ 4 subnets = 64 addresses & 64 + 192= 256...

ThanatosKnight

I think the right answer is E .

The explanation is simple .

Infact , if you plan to have 4 subnets at least ...You need 3 bits regarding the subnetting (The calculation (2³ - 2 = 6 subnets)).

In the last octet , 3 bits mean "11100000" = 224 (the remaining bits are the host bits )

So , you have to choose 255.255.255.224 that is included in the subnet mask 255.255.255.0 (default subnet mask for a C Class Address )

tunerX

C - if you can use ip subnet zero.

E - if you cannot.

E will give you 8 total address ranges, which is more than 4+2. C will only allow 4 address ranges, which is not more than 4+2.

Without subnet zero you automatically toss out the first and last network range.

Cherper

E seems right to me. If you need to have 4 subnets, you need to break the subnet mask down to support that. 255.255.255.224 gives you at least 4 subnets as ThanatosKnight said.

viper75

ThanatosKnight wrote:

I think the right answer is E .

The explanation is simple .

Infact , if you plan to have 4 subnets at least ...You need 3 bits regarding the subnetting (The calculation (2³ - 2 = 6 subnets)).

In the last octet , 3 bits mean "11100000" = 224 (the remaining bits are the host bits )

So , you have to choose 255.255.255.224 that is included in the subnet mask 255.255.255.0 (default subnet mask for a C Class Address )

Well that's the same answer I picked (E) with the same logic using 3 bits and they marked it wrong they say the correct answer is (C). I really don't understand why. Can anyone try to explain this to me???

Cherper

They must be counting subnet zero. That would account for the difference.

viper75

yeah, but if the question doesn't state that then how is one supposed to know???


I think it's a poorly worded question.

Cherper

I agree that it is a poorly worded question, especially since you have to turn on subnet zero to allow it to work.

tunerX

I would always assume that subnet zero is valid unless I am taking a CCNA exam.

tunerX

Cherper wrote:

I agree that it is a poorly worded question, especially since you have to turn on subnet zero to allow it to work.

Any release after version 12.0 has "ip subnet zero" as the default. If the CCNA follows suit with all of the other certs then 12.2 and 12.2T are the tested IOS versions so subnet zero is allowed.

http://www.cisco.com/en/US/tech/tk648/tk361/technologies_tech_note09186a0080093f18.shtml

Cherper

You are right on the ip subnet zero in the latest versions on by default. I thought that it was on by default, but I couldn't remember for sure.

I guess that the question is not poorly worded if the basis is 12.0+.

viper75

tunerX wrote:

Cherper wrote:

I agree that it is a poorly worded question, especially since you have to turn on subnet zero to allow it to work.

Any release after version 12.0 has "ip subnet zero" as the default. If the CCNA follows suit with all of the other certs then 12.2 and 12.2T are the tested IOS versions so subnet zero is allowed.

http://www.cisco.com/en/US/tech/tk648/tk361/technologies_tech_note09186a0080093f18.shtml

According to Todd Lammle's Sybex 640-801 book at the top of page 107 he states,

"In production, you can use 1 bit assigning subnets. This is called subnet-zero. But know that Cisco doesn't consider subnet-zero valid on any of their certification exams!"

tunerX

Unless you get it from a Cisco Site then you should assume subnet zero is valid. Lammle's books were good during the 407 and 507 days but it seems that he just updates books and resells them without much editing and validation.

ThanatosKnight

After I checked the Cisco book , I think I found the right solution .

Infact, if you consider the question as a Cisco question ... The right answer was "E" ...
(Please, refer to page 115 on "CCNA ICND Exam Certification Guide")...However, if you consider It as a "normal" question...The right answer is C , because the zero subnet is also can be used and It has more priority than the others.

Strange question ,but It has two answers ... Very strange world for very funny networkers.

garv221

Trick question, but I add the zeros in & subtract at the end, this question didn't specify.

viper75

I figured that they made some kind of mistake with this question. I'm going to send them an email. This question really threw me off.


Thanks to all for helping me figure this out.


This is a great place!!! CHEERS to the WebMaster!!! Next round is on me.

Webmaster

viper75 wrote:

I figured that they made some kind of mistake with this question. I'm going to send them an email. This question really threw me off.

Thanks to all for helping me figure this out.

This is a great place!!! CHEERS to the WebMaster!!! Next round is on me.

I figured you had a good reason for posting a PrepLogic question word-for-word It will cost you to get me tripple-drunk though Seriously though, you're welcome and I hope you still like it regardless of this question.

viper75

Yeah Johan...I still like it. Use it every day.

fondue

Your organization is using the 198.133.219.0/24 address space for it's Internet presence systems. The IP address plan provides for 4 subnets within the given address space. What network mask are these host in your organization using?

Don't be fooled by the trick questions.

Read the question again. It says 4 subnets, not 4 "usable" subnets. The correct answer is 255.255.255.192.

Drakonblayde

Yeah, but the confusion results from the fact that it's CCNA prep material, and for the CCNA, first and last subnet aren't considered usable. CCNA isn't that tricky, so the question really doesn't have a place, IMHO

kik_azz

Is it possible for a public IP address to be subnetted???