Subnet Zero Question Host -1 or -2

Haydrion

Hello Everyone,

My teacher said to me if subnet zero is on you must do -1 by the host and not -2 .. I search it in my books from Todd Lammle and Odom but I can't find this. So what is true ?

Regards
Danielle (Netherlands)

EdTheLad

Subnet Zero enabled allows you to use the whole address range. For example if you have the network 192.168.6.0/24 and you wanted to borrow 4 bits from the host portion to use as subnets, you have the following subnets:

192.168.6.0/28
192.168.6.16/28
192.168.6.32/28
192.168.6.48/28
"
"
"
192.168.6.240/28

A total of 16 subnets, the first subnet 192.168.6.0/28 is subnet zero. If the command "no ip subnet zero" is configured, you are not allowed to use the first subnet. Therefore if you look at the maths, we borrowed 4 bits from the host.
2^4 = 16 subnets, if subnet zero is not allowed 2^4 = 16 - 1 =15.

Haydrion

Thank you for your answer, but with all the practice exams I did there was no message with subnet zero is on or off, so what should I do always -2 or if nothing is said always -1

EdTheLad

Subnet Zero is on by default, all ones subnet is on by default, don't minus anything. So if you have 4 bits its 16 subnets, forget about subnet zero for the exam.

Haydrion

EdTheLad wrote: »

Subnet Zero is on by default, all ones subnet is on by default, don't minus anything. So if you have 4 bits its 16 subnets, forget about subnet zero for the exam.

Thank you so much for this !

Ltat42a

Assume subnet zero is enabled, unless the question states that it is disabled. If enabled, start at zero. If it is disabled, start at the first subnet after the zero subnet.

hth