If a network address is subnetted, the first subnet obtained after subnetting the network address is called subnet zero.
Consider a Class B address, 172.16.0.0. By default the Class B address 172.16.0.0 has 16 bits reserved for representing the host portion, thus allowing 65534 (216-2) valid host addresses. If network 172.16.0.0/16 is subnetted by borrowing three bits from the host portion, eight (23) subnets are obtained. The table below is an example showing the subnets obtained by subnetting the address 172.16.0.0, the resulting subnet mask, the corresponding broadcast addresses, and the range of valid host addresses
| Subnet Address |
Subnet Mask |
Broadcast Address |
Valid Host Range |
| 172.16.0.0 |
255.255.224.0 |
172.16.31.255 |
172.16.0.1 to 172.16.31.254 |
| 172.16.32.0 |
255.255.224.0 |
172.16.63.255 |
172.16.32.1 to 172.16.63.254 |
| 172.16.64.0 |
255.255.224.0 |
172.16.95.255 |
172.16.64.1 to 172.16.95.254 |
| 172.16.96.0 |
255.255.224.0 |
172.16.127.255 |
172.16.96.1 to 172.16.127.254 |
| 172.16.128.0 |
255.255.224.0 |
172.16.159.255 |
172.16.128.1 to 172.16.159.254 |
| 172.16.160.0 |
255.255.224.0 |
172.16.191.255 |
172.16.160.1 to 172.16.191.254 |
| 172.16.192.0 |
255.255.224.0 |
172.16.223.255 |
172.16.192.1 to 172.16.223.254 |
| 172.16.224.0 |
255.255.224.0 |
172.16.255.255 |
172.16.224.1 to 172.16.255.254 |
In the example above, the first subnet (subnet 172.16.0.0/19) is called subnet zero.
The class of the network subnetted and the number of subnets obtained after subnetting have no role in determining subnet zero. It is the first subnet obtained when subnetting the network address. Also, when you write the binary equivalent of the subnet zero address, all the subnet bits (bits 17, 18, and 19 in this case) are zeros. Subnet zero is also known as the all-zeros subnet.
