Subnetting Question

nezzy456

Hi Guys,

I have a question about Subnetting.
I thought I had it all sussed the other day but I have had my boat rocked.

Basically I understand now that for the Cisco exam we are not allowed to use the 1st and last subnet(correct me if I am wrong please)

So in that case the subnet;
255.255.255.0 (/24)
There is 1 subnet and this is usable

But for the subnet;
255.255.255.128 (/25)
There are 2 subnets but 0 are usable as they are the first and last

And for the subnet;
255.255.255.192 (/26)
there are 4 subnets but only 2 are usable?

Have I understood this properly or have I totally confused the whole thing?

Thanks

dynamik

You're a little off. Why would you be able to use a single subnet (your first example) but neither of the two in your next example?

You're going to want to research subnet zero and the conditions where you can and cannot use the all 0s or all 1s subnets. What training materials do you have? This is usually discussed pretty well.

phoeneous

nezzy456 wrote: »

But for the subnet;
255.255.255.128 (/25)
There are 2 subnets but 0 are usable as they are the first and last

Huh? If 0 are usable, why would /25 networks even exist. I think you are confusing the number of subnets to the number of hosts that can be used. Think of how many hosts you can use and you will understand it more.

nezzy456

Okay,

Basically it was my tutor who messed me up today.

I believed /24 class C network is essentially 1 subnet with 254 host ye?

/25 has 2 subnets with 126 usable hosts in each subnet
/26 has 4 subnets with 62 usable hosts in each subnet

However my tutor tells me you cannot use the 1st or last subnet and if I use them I will be wrong.

Which "logically" leads me to see it like

/25 has 2 subnets (but 0 usable as they are the 1st and last)
/26 has 4 subnets (but 2 usable as you cannot use the 1st and last)

I argued and said so what exactly is the /25 subnet used for then?
And he said you can use it but for the purpose of the cisco exam CCNA I cannot use the 1st and last subnet

See what I mean?

Arggghhhhhhhh

EdTheLad

Your tutor is wrong, you should tell him to get with the times.Subnet zero is enabled by default on current ios, its been like this for years.As a previous poster mentioned read about subnet-zero, go to the search engine on this site and search subnet-zero, im sure you will get loads of hits.

BennyLava

Check out this article:
Subnet Zero and the All-Ones Subnet [IP Addressing Services] - Cisco Systems

My guess is that for the exam they will tell you whether or not subnet zero is usable if its relevant to the question, but if they don't mention it I would assume that it is usable.

Stotic

Yea, your tutor is telling you the wrong information, or just confused himself. Your initial perception is correct.

nezzy456

Thanks guys, I was really worried that what I had learned previously was totally wrong.

Cheers

luke_bibby

The zero subnet (first subnet) and broadcast subnet (last subnet) are reserved (not usable) if:
- The no ip subnet-zero global configuration command has been configured
- The routing protocol is classful (e.g. RIP1)

The two subnets are NOT reserved (usable) if:
- The exam question says nothing about it
- The ip subnet-zero global configuration command has been configured (default)
- The routing protocol is classless (e.g. OSPF)

Straight from the Cisco Press books

phoeneous

nezzy456 wrote: »

Okay,

Basically it was my tutor who messed me up today.


/25 has 2 subnets (but 0 usable as they are the 1st and last)
/26 has 4 subnets (but 2 usable as you cannot use the 1st and last)

Perhaps this will help.

List the ranges of 192.168.1.0/25.

Subnets will go in increments of 128.

First network is 192.168.1.0-192.168.1.127
Second network is 192.168.1.128-192.168.1.255

If 0 are usable then you just disconnected 254 hosts from the network.

How does that make you feel?

Like I said, I think you are confusing what your instructor is trying to explain. If you look at the subnet-zero theory he is probably trying to tell you that you cannot use these networks since they are the all 0's and all 1's networks :192.168.1.0, 192.168.1.255.

Like luke_bibby said, the rules to consider for subnet-zero is:

The zero subnet (first subnet) and broadcast subnet (last subnet) are reserved (not usable) if:
- The no ip subnet-zero global configuration command has been configured
- The routing protocol is classful (e.g. RIP1)

The two subnets are NOT reserved (usable) if:
- The exam question says nothing about it
- The ip subnet-zero global configuration command has been configured (default)
- The routing protocol is classless (e.g. OSPF)

nezzy456

phoeneous wrote: »

Perhaps this will help.

List the ranges of 192.168.1.0/25.

Subnets will go in increments of 128.

First network is 192.168.1.0-192.168.1.127
Second network is 192.168.1.128-192.168.1.255

If 0 are usable then you just disconnected 254 hosts from the network.

How does that make you feel?

Haha, I like your point,

Yes I think I took what he said too literally, as I now realize there are times when its applicable and times when it is not.

I must say this forum is very good, (friendly and fast)

Neeko

I'm not sure about CCNA 4.0 because I'm able to take the 640-801 exam and am studying the CCNA 3.1 curriculum which clearly states that you cannot use the first and last subnets, and the equation 2n-2 is constantly referenced, especially in CCNA1 chapter 10.

What people are saying about this being obsolete is true - but not in CCNA. Instead of just trying to enforce your Godly knowledge on the man, consider that the right answer is dependant on the level of study, and in this case what he is saying is completely correct according to CCNA.

@ nezzy, what you said in your original post shows you understand the subnetting concept. As far as CCNA goes (in my version 3.1 book anyway) the first and last subnets are unusable due to confusion between the overal classful network ID being the same as the first subnet's ID, and the overal network broadcast address being the same as the last subnet's broadcast address.

It's just to avoid confusing people new to the subject, those subnets are used so not to waste addresses. But for now, just go with what your book says otherwise you could get caught out in the exam.

EdTheLad

Neeko wrote: »

But for now, just go with what your book says otherwise you could get caught out in the exam.

I dont understand this logic, believe what you read from a book rather than listening to people who have experience doing the exam?
I haven't come across the perfect Cisco book yet, multiple sources are required and then this forum to point you in the right direction.

nezzy456

EdTheLad wrote: »

I dont understand this logic, believe what you read from a book rather than listening to people who have experience doing the exam?
I haven't come across the perfect Cisco book yet, multiple sources are required and then this forum to point you in the right direction.

I believe what Neeko means Ed is that for the exam I should take the concepts from the cisco site/materials as "gospel" as Cisco are the ones which write the exam and therefore the answers should match their rules.

However for the real world these concepts do not apply in the same way. (Which is where my initial confusion came from, the difference between what Cisco say for the exam and what is true for a real life network).

Again thanks for the help.

kalebksp

Neeko wrote: »

I'm not sure about CCNA 4.0 because I'm able to take the 640-801 exam and am studying the CCNA 3.1 curriculum which clearly states that you cannot use the first and last subnets, and the equation 2n-2 is constantly referenced, especially in CCNA1 chapter 10.

I'm not sure why you would be dispensing advice if you're studying for an old version of the test, 640-802 is the current CCNA exam.

Bottom line: If you're taking a current CCNA exam, unless the question specifically states otherwise assume that the first and last subnet are usable.

Neeko

kalebksp wrote: »

I'm not sure why you would be dispensing advice if you're studying for an old version of the test, 640-802 is the current CCNA exam.

Bottom line: If you're taking a current CCNA exam, unless the question specifically states otherwise assume that the first and last subnet are usable.

I know it is, that's why I made clear I'm studying the old version and therefore it is possible the new curriculum states that the first and last subnets are usable. I don't know if it does, that's why I made it perfectly clear.

If it does, no problem, as long as nezzy knows what the curriculum states he should have no issues. What got me about all the replies here were the way people were reacting like he'd got something drastically wrong, when all it looks like to me is he's got the understanding down fine, but may be misinformed about one part.

Nezzy, I'm glad you see what I'm saying about going by the Cisco books.

What does the Exploration book say about this? I wasn't trying to tell you what your curriculum says or that what I have read is fact, just that my 3.1 version says they are unusable and even though I know they are usable I will go into the exam knowing what the answer is if that question came up. A long as you go with what's in your material you'll be fine.