Subnet Zero

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 two bits of the last octet are binary 1′s or 11000000.

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’t 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