| #bits | mask | 4th octet | prefix | #subnets | @#hosts | subnets' starting octet |
|---|---|---|---|---|---|---|
| 0 | 255.255.255.0 | 00000000 | /24 | 0 | 254 | .0 |
| 1 | 255.255.255.128 | 10000000 | /25 | 2 | 126 | .0 .128 |
| 2 | 255.255.255.192 | 11000000 | /26 | 4 | 62 | .0 .64 .128 .192 |
| 3 | 255.255.255.224 | 11100000 | /27 | 8 | 30 | .0 .32 .64 .96 .128 .160 .192 .224 |
| 4 | 255.255.255.240 | 11110000 | /28 | 16 | 14 | .0 .16 .32 .48 .64 .80 .96 .112 .128 .144 .160 .176 .192 .208 .224 .240 |
| 5 | 255.255.255.248 | 11111000 | /29 | 32 | 6 | multiples of 8 from .0 to .248 |
| 6 | 255.255.255.252 | 11111100 | /30 | 64 | 2 | multiples of 4 from .0 to .252 |
#bits is the number of bits "borrowed" from the host portion,
increasing the size of the mask and the network portion.
Class C default mask is 24 bits (0 subnets and 254 hosts).
@#hosts: the network number address (host portion bits all zeroes)
and the broadcast address (host portion bits all ones) are the
lowest/smallest and the highest/biggest number, respectively, in the
subnet range jand can not be assigned to hosts.
zero subnet (subnet bits all zeroes) and broadcast subnet
(subnet bits all ones)
are "recommended" not to be used. This reduces #subnets for each
prefix by two. These are the first and last subnets for each prefix.
The first is the .0 subnet. The last is the .(256-@#hosts-2) subnet.
Note that with this rule, /25 yields no usable subnets.
| subnet address | 4th octet bits | broadcast address | range of hosts |
|---|---|---|---|
| x.y.x.0 | 00000000 | x.y.x.127 | x.y.x.1 - x.y.x.126 |
| x.y.x.128 | 10000000 | x.y.x.255 | x.y.x.129 - x.y.x.254 |
| subnet address | 4th octet bits | broadcast address | range of hosts |
|---|---|---|---|
| x.y.x.0 | 00000000 | x.y.x.63 | x.y.x.1 - x.y.x.62 |
| x.y.x.64 | 01000000 | x.y.x.127 | x.y.x.65 - x.y.x.126 |
| x.y.x.128 | 10000000 | x.y.x.191 | x.y.x.129 - x.y.x.190 |
| x.y.x.192 | 11000000 | x.y.x.255 | x.y.x.193 - x.y.x.254 |
| subnet address | 4th octet bits | broadcast address | range of hosts |
|---|---|---|---|
| x.y.x.0 | 00000000 | x.y.x.31 | x.y.x.1 - x.y.x.30 |
| x.y.x.32 | 00100000 | x.y.x.63 | x.y.x.33 - x.y.x.62 |
| x.y.x.64 | 01000000 | x.y.x.95 | x.y.x.65 - x.y.x.94 |
| x.y.x.96 | 01100000 | x.y.x.127 | x.y.x.97 - x.y.x.126 |
| x.y.x.128 | 10000000 | x.y.x.159 | x.y.x.129 - x.y.x.158 |
| x.y.x.160 | 10100000 | x.y.x.191 | x.y.x.161 - x.y.x.190 |
| x.y.x.192 | 11000000 | x.y.x.223 | x.y.x.193 - x.y.x.222 |
| x.y.x.224 | 11100000 | x.y.x.255 | x.y.x.225 - x.y.x.254 |