CCNA Subnet Zero Question

Answered Question
Sep 6th, 2007

The network administrator needs to address seven Lans. RIP version 1 is the only routing protocol in use on the network and subnet 0 is not being used. What is the maximum number of

usable IP addresses that can be supported on each LAN if the organization is using one class C address block?

Choice:

A-6

B-8

C-14

D-16

E-30

F-32

The answer is E-30

My question is how come it's 30 ?

I have this problem too.
0 votes
Correct Answer by mohammedmahmoud about 9 years 4 months ago

Hi,

The ip-subnet zero command controls the all zero-subnet ONLY, to be able to use the all 0s subnet on a router - the all 1s subnet is used by default since a while in the IOS, i even think that it was never disallowed - As of Cisco IOS Software Release 12.0, Cisco routers now have ip subnet-zero enabled by default (even before that there was a command called "service subnet-zero").

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

HTH,

Mohammed Mahmoud.

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taylor.alan Thu, 09/06/2007 - 07:06

The answer is 30 because if you subnet a class c address to give you 7 lans you need to borrow three bits from the last octet that will leave you 5 bits for hosts and the formula to work out hosts is 2n-2 where n is the number of bits borrowed for hosts.

Does that make sense?

stalker725 Thu, 09/06/2007 - 18:40

P.S> I know this formula and it doesn't work here. 2^3-2=6 not 7 it also said that subnet-zero is off this is way there is a -2.

oscarbarrios Thu, 09/06/2007 - 07:46

Ok here is the answer;

Remember you need 7 subnets, try 3 bits for de SNW:

2x2x2 that's 8 usables subnets.

For the part of the hosts you have the rest 5 usable bits that?s means:

(2x2x2x2x2)-2 = 30

Regards

stalker725 Thu, 09/06/2007 - 15:42

my only question is why 2^3-2=6 cause you need 7 . I need more explanation on this one please.

glengregory Thu, 09/06/2007 - 19:59

Hi,

The correct answer is C-14. explanation is as follows:-

We need to address 7 lans (means 7 networks / subnets). Since RIP1 is only being used, we cannot use VLSM. We have to use 1 mask for all. Normally, this is simple. Borrow 3 bits (2^3=8) from the class C last octet, which would leave 5 bits for hosts; 2^5-2=30 addresses for each of the 7 lans.

BUT, the question specifically mentions that subnet 0 is NOT being used; which implies that the command "ip subnet-zero" is not in running-config.

Now, our previous calculation, 3 bits (2^3=8) is not valid. It now will be (2^3-2=6) which will not suffice since we need 7 subnets.

So what do we do? We borrow 1 more bit i.e. (2^4-2=14) which meets our 7 lans requirement. What's left for the hosts? 2^4-2=14 hosts per subnet.

Hope this clearly explains the answer.

Glen

stalker725 Thu, 09/06/2007 - 21:20

that's exactly what I thought. for some reason though the "right answer" was 30 go figure out... this whole thing made me confused. some people say it's 30 some say it's 14....

valentin.ene Thu, 09/06/2007 - 21:33

You are wrong. You need 7 subnets with the same network mask (rip v1 doesn't offer support for VLSM). You take 3 MSB bits from last byte, witch means that you have the following subnets (I will take 192.168.0.x class to be more clear) :

192.168.0.0

192.168.0.32

192.168.0.64

192.168.0.96

192.168.0.128

192.168.0.160

192.168.0.192

192.168.0.224

Because you are not allowed to use subnet zero, the 192.168.0.0 subnet will not be use.

The 7 remaining subnets are 192.168.0.x, where x may take this values : {32, 64, 96, 128, 160, 192, 224}, scheme that is covering the requirements.

Because you just borrowed 3 bits from last byte, there are 5 bits for host address, which

is mean that in every subnet you may use a maximum of 30 address per subnet (2^5 - 2).

The correct answer is : 30

stalker725 Thu, 09/06/2007 - 22:31

Well what's your comment on this one then ?

Sybex CCNA Study Guide:

"As I mentioned in the introduction, IP subnet-zero is not a new command, but in the pre-2005

CCNA objectives, Cisco didn?t cover it. This command allows you to use the first and last sub-

net in your network design. For example, the Class C mask of 192 provides subnets 64 and 128

(discussed thoroughly later in this chapter), but with the ip subnet-zero command, you now

get to use subnets 64, 128, plus 0 and 192."

So the first and the last subnet are not used when subnet zero is off. Therefore, we have only 6 subnets or convince me otherwise.

valentin.ene Thu, 09/06/2007 - 22:59

I can't see any reason why you cannot use the latest subnet (in this example 192.168.0.224)

stalker725 Thu, 09/06/2007 - 23:02

this is what I'm asking you why do we use the last subnet when in the book it says we can't use it when subnet zero is off. Please point me to updated info if there is any. I would really appreciate that.

Correct Answer
mohammedmahmoud Thu, 09/06/2007 - 23:44

Hi,

The ip-subnet zero command controls the all zero-subnet ONLY, to be able to use the all 0s subnet on a router - the all 1s subnet is used by default since a while in the IOS, i even think that it was never disallowed - As of Cisco IOS Software Release 12.0, Cisco routers now have ip subnet-zero enabled by default (even before that there was a command called "service subnet-zero").

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

HTH,

Mohammed Mahmoud.

mohammedmahmoud Fri, 09/07/2007 - 01:04

Hi Pavel,

You are very welcomed :) i am glade i helped. When you face some confusing stuff like these, specially when using a non Cisco Press books, always try and search the cisco website and NetPro history for useful CCO documents or comments.

Thank you for using the rating system.

HTH,

Mohammed Mahmoud.

daehong-kim Fri, 09/07/2007 - 03:38

I'm not sure that it's perfect solution.

It's just my opinion.

if C class has eight lan with subnet-zero

then each lan can use 31 respectively

(8 * 32 = 256, but network bit should be subtracted, and without subnet-zero should be subtraced either) 32-2 = 20

stalker725 Fri, 09/07/2007 - 05:08

sorry mate but each network can only use 30 cause 31 is broadcast & 32 is next subnet and 32-2=30 and not 20 :)

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