Subnetting question!

You have been assigned IP address 198.150.25.0/29 and you need to configure the first available IP address on the 1st available subnet for the routers S0 interface and the first available address on the next available subnet for the E0 interface.

Which two commands must you use to properly configure the routers S0 and E0 interfaces?
(Choose two).

A IP address 198.150.25.8 255.255.255.248

B IP address 198.150.25.0 255.255.255.248

C IP address 198.150.25.9 255.255.255.248

D IP address 198.150.25.0 255.255.255.240

E IP address 198.150.25.16 255.255.255.248

F IP address 198.150.25.16 255.255.255.240

G IP address 198.150.25.17 255.255.255.248

H IP address 198.150.25.17 255.255.255.240

I IP address 198.150.25.9 255.255.255.240

My question is... why is subnet 0 not the first available subnet. Surely we're loosing 6 valuable IP addresses here... why cant 198.150.25.1 - 198.150.25.6 be used from subnet 0?

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Comments

dtlokee

Not sure where the question cme from, but if IP subnet zero is on (which is the default) the first subnet would be 198.150.25.0/29. In this case it appears they are following the "no ip subnet-zero" command because the 198.150.50.1/29 ip address is not an option. Based on what's there I wuold say C and G would be the best answers.

In the orignal RFC's that defined IP addressing they excluded subnet zero to avoid confusion of a subnet having the same address as the network itself. This practice should no longer be used due to wasting the addresses, but was the way the CCNA expected you to do it until about a year ago, now they use subnet zero in the CCNA.

Pash

Unless explicity stated in the exam questions, for the current CCNA exam no ip subnet zero is on, the subnet address is not useable in this case.

So yes, looking at the questions C & G are correct.

http://www.cisco.com/warp/public/701/3.html

Note: In the past, there were limitations to the use of a subnet 0 (all subnet bits are set to zero) and all ones subnet (all subnet bits set to one). Some devices would not allow the use of these subnets. Cisco Systems devices will allow the use of these subnets when theip subnet zero command is configured.

From your example there is 5 subnet bit's.

0000 0|001

0000 0|010

0000 0|011

0000 0|100

0000 0|101

0000 0|110

0000 0|111

0000 1|000 - .8

0000 1|001 - Ok!

All subnet bit's are 0's. So it looks like 1-7 isnt useable, nor is .8 of course because it is a subnet address. Remeber 1 up two down for the USEABLE addresses.

so what about .248 +

1111 1|000

All subnet bit's are 1's, indicates not useable, so last subnet IS .240 LAST useable address on that whole network is .246

Netstudent

So what if the question refers to a classless routing protocol like EIGRP or OSPF. Like if it says, "You are configuring an IP scheme for an EIGRP environment, which of these are valid subnets" or something like that. Do we assume the use of the IP subnet-zero??? Thats the last question I will ever have about ip subnet-zero.

kafifi13

Netstudent wrote:

So what if the question refers to a classless routing protocol like EIGRP or OSPF. Like if it says, "You are configuring an IP scheme for an EIGRP environment, which of these are valid subnets" or something like that. Do we assume the use of the IP subnet-zero??? Thats the last question I will ever have about ip subnet-zero.

I'm not a 100 percent sure but i think you are right about that.

totts

Pash wrote:

Unless explicity stated in the exam questions, for the current CCNA exam no ip subnet zero is on, the subnet address is not useable in this case.

So yes, looking at the questions C & G are correct.

http://www.cisco.com/warp/public/701/3.html

Note: In the past, there were limitations to the use of a subnet 0 (all subnet bits are set to zero) and all ones subnet (all subnet bits set to one). Some devices would not allow the use of these subnets. Cisco Systems devices will allow the use of these subnets when theip subnet zero command is configured.

OK... let me get this right as I think I've been getting my terminology mixed up between the '0 subnet' and the 'subnet address'.

Up until now, I thought the command: 'no ip subnet zero' pertained to the the subnet number (I suppose because it has all 0's in it). But (and this is the crucial part), it doesn't does it??? it pertains to the first subnet, am I right?

dtlokee

Unless explicitly stated in the exam questions, for the current CCNA exam no ip subnet zero is on, the subnet address is not useable in this case.

Sorry but the current version of the OFFICIAL COURSEWARE (both INTRO v2.1 and ICND 2.3) specifies that ip subnet-zero is on. Not sure where you heard otherwise, but the Cisco Official Courseware uses the formula 2^s for calculating the subnet bits. Don't go with materials you find here and there that may be dated, go with the Official Courseware.

From RFC 950: (published August 1985)

"It is useful to preserve and extend the interpretation of these special (network and broadcast) addresses in subnetted networks. This means the values of all zeros and all ones in the subnet field should not be assigned to actual (physical) subnets."

From RFC 1878: (published in December 1995)

"This practice (of excluding all-zeros and all-ones subnets) is obsolete. Modern software will be able to utilize all definable networks."

From this we should realize that any OS since 1995 supports VLSM and the all zeros subnet.

All that said it's important for any CCNA candidate to know what the implications are of the "ip subnet-zero" command and the "no ip subnet-zero" command.

Pash

Hmmm, can you print the courseware text which states that dtlokee?

If I am wrong I applogize, but even the cisco press books (for the soon to be expired exams) indicate your subnets are 2^s bits -2. Indicating no ip subnet zero in your schema's.

Yes, all said and done a CCNA candidate should be able to recgonise the difference.

Pash

Netstudent wrote:

So what if the question refers to a classless routing protocol like EIGRP or OSPF. Like if it says, "You are configuring an IP scheme for an EIGRP environment, which of these are valid subnets" or something like that. Do we assume the use of the IP subnet-zero??? Thats the last question I will ever have about ip subnet-zero.

The Question will always state it Netstudent, if it's a SIM you can always use your show commands to find out, and I have seen questions that mention it beign turned on or off in the question text.

But im fairly sure if it is NOT mentioned for a question, then it is off! Meaning you stick with 2^* -2

I actually panicked about ip subnet zero before my CCNA exam, but I was informed several times at my old training college that I would be told either way. Cisco's question pool is tough....but not cruel.

totts

dtlokee wrote:

Not sure where the question cme from, but if IP subnet zero is on (which is the default) the first subnet would be 198.150.25.0/29. In this case it appears they are following the "no ip subnet-zero" command because the 198.150.50.1/29 ip address is not an option. Based on what's there I wuold say C and G would be the best answers.

In the orignal RFC's that defined IP addressing they excluded subnet zero to avoid confusion of a subnet having the same address as the network itself. This practice should no longer be used due to wasting the addresses, but was the way the CCNA expected you to do it until about a year ago, now they use subnet zero in the CCNA.

The question came from Transcender and the correct answer is... C and G.

totts

dtlokee wrote:

Unless explicitly stated in the exam questions, for the current CCNA exam no ip subnet zero is on, the subnet address is not useable in this case.

Sorry but the current version of the OFFICIAL COURSEWARE (both INTRO v2.1 and ICND 2.3) specifies that ip subnet-zero is on. Not sure where you heard otherwise, but the Cisco Official Courseware uses the formula 2^s for calculating the subnet bits. Don't go with materials you find here and there that may be dated, go with the Official Courseware.

From RFC 950: (published August 1985)

"It is useful to preserve and extend the interpretation of these special (network and broadcast) addresses in subnetted networks. This means the values of all zeros and all ones in the subnet field should not be assigned to actual (physical) subnets."

From RFC 1878: (published in December 1995)

"This practice (of excluding all-zeros and all-ones subnets) is obsolete. Modern software will be able to utilize all definable networks."

From this we should realize that any OS since 1995 supports VLSM and the all zeros subnet.

All that said it's important for any CCNA candidate to know what the implications are of the "ip subnet-zero" command and the "no ip subnet-zero" command.

This appears to be a bit of a grey area... I've taken this from the Cisco Press CCNA Intro book, page 348

"Subnet zero or the zero subnet is numerically the first subnet, and its one of the two reserved subnet numbers in a network. (You can use the zero subnet on a Cisco router if you configure the global configuration command 'IP zero-subnet'). Interestingly, a networks zero subnet has the exact same numeric value as the network itself which is one of the reasons that it should not be used. For the purposes of answering questions on the exam about the number of valid subnets in a network, consider the zero subnet unusable unless the question tells you that using it is ok. In real life, do not use the zero subnet if you do not have to".

Webmaster

dtlokee is correct. We had many discussion about this in these forums (for example www.techexams.net/forums/viewtopic.php?t=17350
but it's also in the our CCNA FAQ ) and besides it being logical (based on the RFCs) it's publicly announced by Cisco (see topic below)

I.o.w. all subnets and addresses are usuable 'unless' otherwise stated (and then the 'hint' will be very clear).

www.techexams.net/forums/viewtopic.php?t=21152

[edit: pasted quote below]

Hi. Martin Benson here, I work with the team that creates the INTRO, ICND, and CCNA exams.

The test items on the CCNA exam will tell you whether ip sunbnet-zero is in effect or not. In general, you should assume for the CCNA that all subnets are usable unless the question indicates otherwise. There will always be enough information given in the question to indicate which subnetting scheme is legal for the problem space.

Hope this helps.

Martin

totts

Webmaster wrote:

dtlokee is correct. We had many discussion about this in these forums (for example www.techexams.net/forums/viewtopic.php?t=17350
but it's also in the our CCNA FAQ ) and besides it being logical (based on the RFCs) it's publicly announced by Cisco (see topic below)

I.o.w. all subnets and addresses are usuable 'unless' otherwise stated (and then the 'hint' will be very clear).

www.techexams.net/forums/viewtopic.php?t=21152

[edit: pasted quote below]

Hi. Martin Benson here, I work with the team that creates the INTRO, ICND, and CCNA exams.

The test items on the CCNA exam will tell you whether ip sunbnet-zero is in effect or not. In general, you should assume for the CCNA that all subnets are usable unless the question indicates otherwise. There will always be enough information given in the question to indicate which subnetting scheme is legal for the problem space.

Hope this helps.

Martin

Thanks very much for that... it does and it doesn't help. It helps as it comes from someone in the 'know' and it doesn't as it contradicts a lot of material out there including Cisco Press... taken from CCNA Intro page 348 "For the purposes of answering questions on the exam about the number of valid subnets in a network, consider the zero subnet unusable unless the question tells you that using it is ok"! It also goes on to say that the zero subnet shouldn't be used in real life too. This book is from 2004, perhaps its out of date.

I feel that I still don't know the answer to this and am hoping that they put enough detail in the exam question to be able to decipher it.

Webmaster

totts wrote:

as it contradicts a lot of material out there

What is 'a lot'? Most current material will actually contradict with what is in that one particular Cisco press guide, including the RFCs.

totts wrote:

It also goes on to say that the zero subnet shouldn't be used in real life too.

Regardless of the CCNA exam, that wasn't accurate even in 2004, unless it added somthing about supporting legacy devices.

But really, if you'd go over the discussions in these forums, add up the several resources I listed, add up the RFCs, and the comment from Cisco above, and the fact that ip subnet zero is enabled in the IOS versions explicitely covered on the current CCNA exams, and add a good dosis of common sense, you'll have your answer and there's no need to doubt. And if you know your stuff it's nothing to worry about either. They are not out to trick you and very well realize this is a tricky subject that needs to be tested in a clear manner.

Pash

The 2005 cisco press book also states 2^s -2 for all network schemas. But to be fair this is only while teaching subnetting, not when applying IP address shcemas to network devices...where these extra ip command line options are applied.

I guess only the people who write the questions can confirm.

If someone has or finds a link from cisco's papers confirming this, that would be very helpful. I had a limited amount of search time at work...not successful.

And again without breaking NDA for the exam

....they DO tell you totts...so don't panic just realize what either mean.

Cheers,

dtlokee

Not for nothing, but Cisco Press, Sybex and whatever else are not the official courseware that is used by Cisco. I am telling you what Cisco expects, and it should not be confusing. I am not saying that the Cisco Press book is wrong, it is outdated when compared to the current exam (exam was updated in November 2006). I deliver authorized Cisco training (the stuff the exam is based on) and they use 2^s for subnetting.

If a question wants you to use 2^s - 2 it will be very obvious (eg, they will state it or the answer choice just won't be there)

HTH

Netstudent

thank you Johan, thank you Derek...Can't be anymore clear than that.

totts

OK, thats great, I'm happy with this area now. Many thanks for the information and comments.

Pash

I am glad you two are clear...I am not

I will have a further look tonight when I have time to see if i can find anything that remotely resembles a clear picture. Btw im not dissing anyone when saying this........you have to admit it is very etchy

Cheers,

totts

Pash wrote:

I am glad you two are clear...I am not

I will have a further look tonight when I have time to see if i can find anything that remotely resembles a clear picture. Btw im not dissing anyone when saying this........you have to admit it is very etchy

Cheers,

My approach now is to go with whats been said, ie the zero subnet is to be used, but to also be mindful that this is an 'etchy' area, so use common sense when answering questions on the subject. I think that that is the general message thats been put across. Thanks again.

Netstudent

Pash wrote:

I am glad you two are clear...I am not

I will have a further look tonight when I have time to see if i can find anything that remotely resembles a clear picture. Btw im not dissing anyone when saying this........you have to admit it is very etchy

Cheers,

You already have the CCNA so you should know exactly what the test is going to throw at you.

Pash

Posted by: mcbenson - Jul 17, 2006, 2:34pm PST

Hi. You will need to read the question stem carefully to determine whether subnet-zero is usable. We realize that there maybe confusion regarding this, so we give all the information you require in questions.

That was the only "recentish" post I could find on the matter after 10 minutes of searching. Couldnt actually find that post quoted in the above link...might not be there anymore or Im crap at searching

Good on you totts, btw your living in manc land now and you are from essex? Which part of essex you from?

Cheers,

Pash

Netstudent wrote:

Pash wrote:

I am glad you two are clear...I am not

I will have a further look tonight when I have time to see if i can find anything that remotely resembles a clear picture. Btw im not dissing anyone when saying this........you have to admit it is very etchy

Cheers,

You already have the CCNA so you should know exactly what the test is going to throw at you.

Im not confused by the subject in the slightest fella, I thought I made that clear....I am confused by how Cisco represents this subject in the CCNA exam. And apparently so are others above....i see inconsistencies!

Anyway,

I don't usually like putting lead weights on things that puzzle me, but im hungry....pizza for me

Netstudent

Oh I see...I have noticed some slight ambiguity in cisco's methods as well. hopefully when testing time comes, the inconsistency won't lead me astray.

totts

Pash wrote:

Posted by: mcbenson - Jul 17, 2006, 2:34pm PST

Hi. You will need to read the question stem carefully to determine whether subnet-zero is usable. We realize that there maybe confusion regarding this, so we give all the information you require in questions.

That was the only "recentish" post I could find on the matter after 10 minutes of searching. Couldnt actually find that post quoted in the above link...might not be there anymore or Im crap at searching

Good on you totts, btw your living in manc land now and you are from essex? Which part of essex you from?

Cheers,

Good stuff, I think we got that one sorted! Pash, I'll send you a message abt Essex!

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