subnetting exam questions: classful vs. CIDR

I don't want to beat a dead horse, but I found this thread really interesting. I just started using subnettingquestions.com to boost my subnetting speed, and got the first couple of questions wrong because of the same reasoning as the thread's original poster.

It got me thinking:

Why do Cisco exams present subnetting questions in terms of classful addressing (similar to the debate in the thread), while in the real world, CIDR and VLSM make the answers to those questions incorrect?

Given that CIDR has been used since 1993, doesn't that mean that the methodology required to "correctly" answer exam questions gives a student false information?

If I'm showing my ignorance here, please correct me.

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Comments

EdTheLad

I have a similar question, why does mother nature teach babies to crawl, shouldn't they just walk from day one.I haven't crawled sober for 30 years, why did i have to bother learning to crawl in the first place.

katachi

EdTheLad wrote: »

I have a similar question, why does mother nature teach babies to crawl, shouldn't they just walk from day one.

I wholeheartedly agree with you that it's imperative for students to understand the building blocks of a discipline in order to master it.

But continuing with your analogy, EdTheLad, the exams present a situation closer to your elementary/primary school track-and-field coach giving try-outs by only testing whether or not runners can crawl, not run. For strictly philistine candidates, this might give the impression that track-and-field is made up of crawling events. (Cheers, Ed

)

I'm not trying to provoke anyone, but...is the reason for this type of single-sided test question really just to prove that "yes, you know the material the book told you to learn, despite the fact that your real-world experience teaches you otherwise", with no mention that things might function differently?

EdTheLad

I don't get what you mean regarding real world experience telling me otherwise. Classful or Classless i never mix anything up here.I will always look at an address from a classful perspective and then depending on circumstances switch to classless if required.

Looking at a practical example:

You have a RIPv2 network, its divided into two domains each with a separate networks A&B, A =10.1.1.x/24 and B=192.168.1.0/24. Department B calls you and asks why they are receiving a 10.0.0.0/8 route, they prefer to have longer prefixes.This /8 jingles no bells with you as you don't understand classful addressing.You ask a colleague, he knows classful addressing and straight away asks if auto-summary is enabled.

Later Department B calls again, they are trying to configure a summary address on the interface connected to Dep A, it keeps throwing up an error message.The summary they want to use is 192.168.0.0/16, why wont it work?You ask your colleague again, he explains how rip cant summarize with a mask shorter than the natural mask for class C. Natural class C mask you ask? This is around the time your boss arrives and says goodbye.

gojericho0

you almost have to look at classful and classless in terms of addressing and routing

classful/classless address

Classful address is the strict adherence to the A,B,C and their respective subnet masks

Classless address does not put a restriction on the SNM and allows you to use any mask you desire.

classful/classless routing

Classful routing does not send a subnet mask with its updates, but still allows you to subnet because you are staying within the classful network boundry. If you compare the major network with the SNM in binary all 1's classful would still be matching. The only catch is you would have to use the same subnet mask for all subnetworks of that major net because the SNM is not being sent in the routing update.

Subnetting CIDR is also not possible with a classful protocol. Like in ed's example you wouldn't be able to summarize a 192.168.0.0 /16 because you would be going outside the major net boundry. Your binary comparison would have your SNM 1's would not match up with the classful networks.

Classless routing does send the SNM and therefore all the shortcomings (no VLSM or supernetting) of the classful protocol are addressed.

katachi

Am I correct in assuming that nearly every network today uses classless addressing and routing?

If this assumption is not correct, then I have been dutifully put in my place.

EdTheLad

My examples were in regard to classless routing, i was trying to point out that just because you use classless routing does not mean you will not need to know classful addressing.

Forsaken_GA

katachi wrote: »

Am I correct in assuming that nearly every network today uses classless addressing and routing?

If this assumption is not correct, then I have been dutifully put in my place.

It's a safe assumption, but you still need to know both sides of it. Legacy equipment and installs do exist, and it's hard to understand the current state of things without understand what went before, and why the evolution to CIDR was necessary.

kryolla

they should of been more specific like given this classful address how many networks and hosts. Nobody uses RIPv1 anymore and turning off auto-summary is like 2nd nature but you still have to learn classful addressing and subnetting. If you know VLSM then classful is even easier.

katachi

Will Cisco exams be so explicit when asking the examinee to determine the number of subnets and hosts from a given address and mask? Should you assume classful or classless by default?

skrpune

katachi wrote: »

Will Cisco exams be so explicit when asking the examinee to determine the number of subnets and hosts from a given address and mask? Should you assume classful or classless by default?

I'm not sure how it is for Cisco exams, but in terms of the Network+ exam, I'm pretty sure that classful addressing is assumed when you are given an IP address and a full subnet mask address, and classless addressing is assumed when it is presented as an ip address /#bits used for subnet mask.

SO, for example:
- 172.31.0.0 255.255.255.128 would be classful, and if it were presented as
- 172.31.0.0 /25 it would be treated as classless.

Like I said though, I'm not sure if this is a case where there is a specific notation for Network+ vs Cisco, but all my N+ sources do say that the difference lies in how the subnet mask is expressed and you can assume classful vs classless by looking for that little forward slash...

katachi

Gotcha. I think that EdTheLad gave some very sage advice both for the exam room and out in the field:

EdTheLad wrote: »

I will always look at an address from a classful perspective and then depending on circumstances switch to classless if required.

Cheers, Ed

klokwyze

I was in the same boat as katachi before reading this thread. I have now attained enlightenment.

Damn this thread is almost 4 years old!