Question on Zero-subnet in the test

Hi Experts.

I am confused.

I have a question that says assign the 3rd usuable subnet to the serial interface of the router but note the sub zero subnet is not usable. The network is / 240. this is just an example. Assign the last usable IP from the 3rd usuable subnet.

I know the first networrk is 0
2nd is 16
3rd is 32

What is my answer and shall i still consider the network 0 even though the question tells me to consider subnet zero is not usuable.

Habib ZNetwork AnalystAsked:
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AkinsdConnect With a Mentor Network AdministratorCommented:
The 1st subnet = Zero subnet assuming that the command "ip subnet-zero" is enabled.
ip subnet-zero is enabled by default on every device
the question says, ip subnet-zero is unusable meaning it was disabled with the command
"no ip subnet-zero"
This means that the ip subnet "" is not valid

Please confirm the slash notation used
Did you mean
Or which would = /28

The answer to this will determine what the network range is

I will assume you meant /28

If that is the case
Your 1st subnet is -
2nd Subnet is -
3rd Subnet is -

Your last assignable address in the 3rd subnet would therefore be

For experimental purposes
issue the command "no ip subnet-zero
then go on any physical or virtual interface and attempt to assign any ip address from -
The switch will reject the ip because all of those belong to ip subnet-zero
it depends on what device you are using this zero subnet because some of the devices are using the zero subnet.
perolinConnect With a Mentor Commented:
I agree with Akinsd.
Your example is not valid.  Eder it is or /28.

Use IP Calculators, that can be found several in the Internet.

If your example is a IP Range you received from your ISP, then it is common, that you cant use the first and the last IP of the range. First is your NetworkID, Last your Broadcast.  ISP Information to you  (of Corse is private)
Hosts/Netz:    14
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AkinsdNetwork AdministratorCommented:
Assign the last usable IP from the 3rd usuable subnet.

Answer =

Pardon my typo above

Your 1st subnet is -
2nd Subnet is -
3rd Subnet is -

Your last assignable address in the 3rd subnet would therefore be
the .63 is a broadcast address and is not assignable
One does wonder why someone is teaching IP subnetting/addressing techniques that are 15 yrs out of date...
AkinsdNetwork AdministratorCommented:
It is obsolete and I agree, but it is still valid and available for whoever wants to use it.

I think the question is focused on testing the student's understanding of ip subnet-zero.

You will be amazed at how many Engineers lack the understanding.

I wouldn't write the question off though.

Just to recap
With ip subnet-zero enabled, your first usable subnet is -
With ip subnet-zero disabled, your first usable subnet is -
I wasn't writing off the question, I answer a number of questions on EE that come with the advice "but don't do it that way"...  I just hope whoever's teaching the class provides the caveat that ip subnet-zero should not be used in anything other than a historical example...  

As to being amazed, I tend, rather, to be jaded on the lack of ability/understanding even at the carrier level...  Oh well...
Habib ZNetwork AnalystAuthor Commented:
Thank you all.

I know the difference now with your superb explanation.
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