Silly Subnetting

bmauro

So I have the Intro test on May 30th - about a week and a half away. I've been studying and reviewing my butt off and spending some time over at http://supnettingquestions.com and I just had two or three questions that are questioning my subnetting methods.

Here is one question that I really hope is just a mistake.

Question: How many subnets and hosts can you get from the network 192.168.187.0/28?

Answer: 16 subnets and 14 hosts

I've been going with the 2^number of subnets - 2 formula, and for the above question I came up with 14 subnets and 14 hosts.....I was very surprised when I saw that 16 in there. /28 is equal to 255.255.255.240 - thats four bits for subnets, four bits for hosts. 2^4 - 2 = 14......

I went back to the website and did see "subnet zero allowed is set to TRUE" and to be honest I don't really know what that means. Is that saying that they are allowing the two reserved subnets, and thats where the 16 is coming from?

I just ran over to cisco.com to get some information on zero subnet and it looks like thats the case - which is starting to scare me abit since the way I've been subnetting for the past year is incorrect (slightly).

Which way should I be subnetting for the CCNA?

Thanks for any help!

bmauro

Ok - I did what I should have done in the first place and did a search on subnet zero in the forums and got a lot of answers.

But one question is still lingering in my mind.

The general opinion is that on the CCNA they will tell you if subnet zero is being used - so I can ASSUME that if it is not mentioned anywhere that subnet zero is not being used? The only reason I ask is that since IOS 12.0 it is turned on by default.

Thanks guys

eurotrash

Don't take my word for it, but I believe that Cisco doesn't use the subnet zero in their exams (unless specified), while Microsoft does.

Humper

With subnet zero on you can you the first and last subnet that's why to figure out the subnets it's 2^n = # of subnets. While with no subnet-zero you DO not use the first and last subnets that's why they give you the formula 2^n - 2 = # of subnets. Now for the exam I believe they will tell you whether or not subnet zero is on or off, or give you a hint of some sort.

All you have to remember is:

subnet-zero = 2^n (all subnets are available)

no subnet zero = 2^n - 2 (first and last are not usable)


EASY!

lordy

If I remember correctly subnet zero should NOT be used in Cisco exams unless specified.

I know it's odd cause it is enabled since 12.0 but it was that way when I did CCNA about a year ago.

dmaftei

bmauro wrote:

I've been going with the 2^number of subnets - 2 formula...

Do youself a favor and forget about 2^n - 2, it's brain damaging... The number of subnets is always 2^n; whether or not you throw away the first and last subnets is a different matter.

bmauro

Thanks for all the input everyone.

ONE last question on subnet zero...promise :P

I just recently had a practice question that asked how many subnets you would have with a /28 subnet mask on a Class C address running EIGRP. No mention anywhere on if you were using subnet zero or not.

Of coarse two of the options are
16subnets - 14 hosts
14subnets - 14 hosts

Not seeing subnet zero mentioned anywhere I went with the 14 - 14 answer and it was wrong. The answer said something along the lines that since EIGRP uses VLSM that you don't reserve the zero subnet or the one subnet. News to me.

I know that this probably won't affect me too much during the test - I just want to understand this. Thanks for any help.

dublin_101

bmauro wrote:

Question: How many subnets and hosts can you get from the network 192.168.187.0/28?

Answer: 16 subnets and 14 hosts

i haven't done the final yet, but i've come across it quite a few times and i ask my teachers many many questions.......basically ip subnet-zero is enabling the subet to use the 0s and 1s as a subnet also.........therefore in such a case a subnet borrowing four bits would in fact have 16 usable subnets!...

very interesting as they teach us that 4 bits = 16 minus 2 (network and broadcast), and then later on they tell us, "well, actually you can use more but you need to command it to do so!"

forbesl

dublin_101 wrote:

very interesting as they teach us that 4 bits = 16 minus 2 (network and broadcast), and then later on they tell us, "well, actually you can use more but you need to command it to do so!"

Are these these same instructors that told you that serial wan connections are "technically" broadcast domains?

You haven't had to "command" ip subnet-zero for quite a while now. Tell your instructors to research IOSs 12.0 and above. That's default with those IOSs....in other words, you have to "command" it NOT to use subnet-zero and the all ones subnet.

Here's how you "command" it not to use subnet-zero:

Say DON'T DO THAT!!

If that doesn't work, try turning the router upside down and shaking it vigorously several times. If it still doesn't work tell your "knowledgeable" instructors. They may be able to make it stop by using a better command voice.

dublin_101

forbesl wrote:

Say DON'T DO THAT!!

If it still doesn't work tell your "knowledgeable" instructors. T/quote]


funny, but i'm not that silly to tell me teachers that they are wrong......even if i can prove they are, i still won't do this!....too risky as i still need them!....

Humper

forbesl wrote:

Say DON'T DO THAT!!

If it still doesn't work tell your "knowledgeable" instructors. T/quote]


funny, but i'm not that silly to tell me teachers that they are wrong......even if i can prove they are, i still won't do this!....too risky as i still need them!....

Well you should correct them because they are going to teach future students, and they are also going to get incorrect information.

EdTheLad

ModemHumper wrote:

Well you should correct them because they are going to teach future students, and they are also going to get incorrect information.

Its not incorrect information.Subnet-zero can still cause problems on old legacy networks.The fact is the instructor should teach the origins and this includes classful protocols,he should explain why subnet zero was used and why it isnt used anymore.The problem isnt so much what he's teaching its how he's teaching.