Subnet question

mike3

I've been practicing subnetting at http://www.subnettingquestions.com . However, I noticed when it's asking you questions like "how many subnets and hosts can you get from 172.168.7.0 255.255.240.0" . Well base on their site you'd get 16 subnets and 4094 hosts". I was wondering though, I've seen from other sites that the CCNA test will say that their is 14 subnets and not 16.

Is this true?

scheistermeister

That depends. If you do not use subnet 0 or what many consider a broadcast subnet then you get 14. On a (older) cisco router you needed the command ip subnet-zero to use the 0 subnet. It is something they don't really tell you about till CCNP 1. There is a lot of stuff they don't tell you really to make it easier on you.

It is mainly an older way of thinking and doing things that says there are only 14, but really there are 16 use-able subnets there.

mike3

So how should I answer these questions on the CCNA? Would it be 16 or 14 subnets?

nicklauscombs

for the CCNA ip subnet zero is usuable unless it says otherwise or has the no ip subnet zero command in the question

networker050184

I would use subnet zero unless other wise stated. Don't worry the questions will be clear!

Pash

I think the actual course material and exam spec all point towards ip subnet zero on nowadays, its the same for all routers with IOS > 12.x in the real world anyhow.

scheistermeister

I am not sure on the new CCNA but on the old on it would be if they never mentioned anything then you could not use subnet 0 if they wanted it included they said "The command ip subnet-zero has been used."

If it is a straight question like "How many subnets are there?" it would be 16 with or without since they are still there, it didn't ask if they were usable or not.

If it asks how many usable subnets there are and says nothing about ip subnet-sero then I would say 14.

nicklauscombs

scheistermeister wrote:

If it asks how many usable subnets there are and says nothing about ip subnet-sero then I would say 14.

I will disagree with this correct me if i'm wrong though, from my understanding ip subnet zero is considered always on now unless specified

scheistermeister

nicklauscombs wrote:

scheistermeister wrote:

If it asks how many usable subnets there are and says nothing about ip subnet-sero then I would say 14.

I will disagree with this correct me if i'm wrong though, from my understanding ip subnet zero is considered always on now unless specified

That's why I said on the old CCNA stuff. I went through the Net Acad as the last class to be taught the old material instead of the new and also took the tests for the old retired exams in January of this year. I haven't looked through the new stuff too much yet, so I wouldn't knw how it is being taught now.

nicklauscombs

gotcha

mike3

Okay well I will answer as if subnet-zero is on by default.

I will be happy to get this exam out of the way after this week.

aragoen_celtdra

scheistermeister wrote:

If it asks how many usable subnets there are and says nothing about ip subnet-sero then I would say 14.

This would have been true of the old exam. However, like Pash said, the ip subnet zero has been on by default since IOS 12.0. And yes, subnet zero IS a usable subnet. Articles may dissuade you from using it because it may cause problems with legacy equipment, but yes, it is usable.

Here, check this out (at the bottom of the page:

It should be noted that even though it was discouraged, the entire address space including subnet zero and the all-ones subnet have always been usable. The use of the all-ones subnet was explicitly allowed and the use of subnet zero is explicitly allowed since Cisco IOS Software Release 12.0. Even prior to Cisco IOS Software Release 12.0, subnet zero could be used by entering the ip subnet-zero global configuration command.

So if nothing is mentioned, assume that it is on and should be counted.

mike3

Thanks for the advice guys.