Subnet Zero
IP subnet zero questions seem to pop up on various
network discussion groups every few days. Some people are not
clear about what it is and how it will affect them in interviews and
exams.
What is Subnet Zero?
When you are calculating subnet masks the result
of your calculations generates various subnets. Each subnet
starts with a number so if you use the example in easy way to subnet
you will see that your subnet addresses are:
192.168.1.0
192.168.1.64
192.168.1.128
192.168.1.192
Subnet zero is the subnet which has all binary 0's
in the address. So for the number 192.168.1.0 you have in the
last octet eight zeros for the subnet number or 00000000. The
last octet has all 1's in the subnet part. Please re-read the
easy way to subnet section again but remember that we are stealing
three bits from the last octet to make a subnet. If you write
out the address 192 for the last subnet above you would see that the
first three bits of the last octet are binary 1's or 11100000.
The same actually goes for the first subnet.
I know I said that there were eight binary 0's in it but only the
first three count for the subnet, the last five are for the hosts on
that subnet. If subnet zero is not allowed you will always
lose two subnets so in the below example if your subnet mask is
255.255.255.224:
192.168.1.0 - You
can't use this subnet as it is all 0's in the subnet
192.168.1.64
192.168.1.128
192.168.1.192 - You
can use this subnet as it is all 1's in the subnet
If you can use subnet zero you get this:
192.168.1.0 - You can use this subnet
192.168.1.64 - You can use this subnet
192.168.1.128 - You can use this subnet
192.168.1.192 - You can use this subnet
So Can You Use Subnet Zero?
The answer is 'yes you can.' Unless you are
told otherwise you should presume that subnet zero is always
allowed. In an IT exam it will usually tell you if you are not
permitted to use it but to prevent it's use is a bit old fashioned.
Subnetting Secrets Works Hi Paul,
| Thank you
for your e-mail. The books and videos are excellent.
Very easy to follow and understand. I have an interview
Friday for a network engineer position. This will help
me to refresh my networking skills and knowledge.
Daniel Norris - Unix Administrator |
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