Subnet zero question

Working on MY ICND1 exam, and fine tunning my subnetting subject.
I am pretty good with subnetting, but after reading a few different materials, i was kind of confused.

If subnet zero not available (where square is borrowed subnet bits)2²=available subnets. If subnet zero is available 2²-2=Available subnets.
But no matter what is subnet zero is, available host is (where square is host bits) 2²-2= available host.

So i am confused with host portion of the calculation. Subnet zero has nothing to the with host portion, right?
I am not actually confused, i just want to make sure.

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Comments

MrBrian

mella060 wrote: »

I don't see the big issue here. If the command is listed on the router, then you can use the 1st and last subnets. If it says no ip subnet-zero then you cannot use the 1st and last subnets.

The problem with that is that it's wrong lol. With "no ip subnet-zero" you CAN use the all ones subnet.

Hopefully aspiring CCNA'ers won't get questions on the exam about that because I see what it says in Lammle's book and it's not correct. It might be one of those things where you need to answer like the book says though, not sure.

mapletune

i doubt that will be asked on the exam... =/ like... ip or no ip zero, -0, -1, -2 -3.... that's so lame. (no offense)

my experiences with Cisco exams have all been very good, very practical

that's why i doubt they'll play word games like that.

Michael2

I re-took the ICND1 test in the practice exams section of this site and there is a question on this topic. It has a class B address and you are asked how many subnets and hosts you get if you subnet it out using the mask 255.255.224.0 and the ip subnet-zero command is not enabled. The answer is 6 subnets with 8190 hosts. Either that thing is out of date, or the people on here that say you can't use the broadcast range are correct.

TehToG

You CAN use the all ones subnet regardless. This. is. a. fact.

There was an old RFC that recommended you didn't use the all ones subnet (which has now been recinded suggesting the use of both subnets is a good idea), and infact unless you're stuck for space you will never need to use it. It's like how the book says to use /30 subnets for point to point links, Tell me why I can't use a /31 instead?

Now the ICND1 I took a few months back asked that I knew what No IP Subnet did but it DID NOT ask me to subnet something with it inforce.

NetworkVeteran

Tell me why I can't use a /31 instead?

Right on. As an aside, I see /31s often in the wild. It happens to be one of the easiest masks to misconfigure, and I've caught more than one person goofing when using it.

TehToG

If configured properly there are no issues though. Most people just don't use them to avoid possible problems which is identical to this ip subnet zero issue.