IP Subnet zero

IP Subnet zero

In the old days, IP Subnet-zero was not default in routers. i.e. Some older technologies don't allow the use of this first subnet.
The first IP is used as subnet and last IP as the broadcast.
It has nothing to do with the network and broadcast addresses.

Example: 1

Let's take the 10.0.0.0/8 network, and use  3 bits for subnetting.
That is 255.0.0.0 will be 255.11100000.00000000.00000000
                         = 255.224.0.0
 
Subnets = 2^3 - 2 = 254.
Valid subnets = 256 – 224 = 32. 0, 32, 64, 96, 128, 160, 192, 224. 
Block Size = 2^5 = 32.
So, by using the first 3 bits, we can create 8 subnets,
they would be:
10.0.0.0/11    first subnet
10.32.0.0/11
10.64.0.0/11
10.96.0.0/11
10.128.0.0/11
10.160.0.0/11
10.192.0.0/11
10.224.0.0/11  last subnet

In binary, the first 3 bits of the 2nd octet for these networks
would look like this:
000   This is the first subnet 
001
010
011
100
101
110
111   This is the last subnet in the range

If the subnet 0 is not allowed, it actually means that we can't use subnet 000 and subnet 111 in this example. (10.0.0.0/11 and 10.224.0.0/11).
The calculation for subnets, without subnet 0, is 2^(power of borrowed bits) – 2 = Amount of Subnets.

Example: 2

When we do VLSM, say we break 192.168.1.0/24 to four subnets
192.168.1.0/26 – can’t be used as 192.168.1.0/24 is already named
192.168.1.64/26
192.168.1.128/26
192.168.1.192/26 – can’t be used as it contains 192.168.1.255/24
which is already used as broadcast.

IP subnet-zero, allows for the first and last subnet in a
subnetted network to be used.

IP Subnet zero