/27 Subnet Mask is a subnet mask that divides a large network into multiple smaller subnetworks. Each of the subnetworks will have a maximum of 32 IP address ranges. The first IP address in this range is used as the network address, while the last IP address in this range is reserved as the broadcast address. Hence, there are 30 usable IP addresses in a /27 subnet mask.
For example, if we subnet the network 192.168.2.0/24 with a /27 subnet mask, the first subnet will have the network address 192.168.2.0, a usable IP range from 192.168.2.1 to 192.168.2.30, and the broadcast address 192.168.2.31. The second subnet will have a network address of 192.168.2.32, a usable IP range of 192.168.2.33 to 192.168.62, a broadcast address of 192.168.2.63, and so on (till the eighth subnet).
Here is a table of a network subnetted with a /27 subnet mask:
| Network address | Usable IP addressess | Broadcast address |
|---|---|---|
| .0 | .1-.30 | .31 |
| .32 | .33-.62 | .63 |
| .64 | .65-.94 | .95 |
| .96 | .97-.126 | .127 |
| .128 | .129-.158 | .159 |
| .160 | .161-.190 | .191 |
| .192 | .193-.222 | .223 |
| .224 | .225-.254 | .255 |
/27 Subnet Mask Usable IP Address
In /27 subnet mask, there are 32 ranges of IP addresses; the first one in the range is reserved for a network address, and the last IP address in this range is reserved as a broadcast IP address. Hence, there are 30 usable IP addresses in the /27 subnet mask.
/27 Subnet Mask Dotted Decimal Notation
The dotted decimal notation for a /27 subnet mask is 255.255.255.224. This representation is gotten by setting the first 27 bits of the 32-bit IPv4 address to ‘1s’, setting the remaining 5 bits to ‘0s’, and converting from binary to decimals.
Subnetting Class C Network with /27 Subnet Mask
When Class C network is subnetted with a /27 subnet mask, 3 bits are borrowed to the network bit, leaving 5 bits for host bit.
Hence, the number of subnet obtainable when subnetting class C network with /27 is gotten as follow.
2^3 = 8
Thus, subnetting a Class C network with a /27 prefix length divides the Class C network into 8 subnetworks, each having 32 IP address ranges.
Subnetting Class B Network with /27 Subnet Mask
To subnet a Class B network with a /27 subnet mask, 11 bits need to be borrowed and added to the network bits.
Hence, the number of subnet obtainable when subnetting class B network with /27 is gotten as follow.
2^11 = 2048
So, a /27 subnet will divide a Class B network into 2048 subnetworks. Each subnetwork will have 30 usable IP addresses.
Subnetting Class A Network with /27 Subnet Mask
To subnet a Class A network with a /27 subnet mask, we need to borrow 19 bits from the host bits and add them to the network bits.
So, the number of subnetworks obtainable when we subnet class A network with /27 prefix length is gotten as follow;
2^19 = 524288
This means that a /27 subnet will divide a Class A network into 524288 subnetworks. Each subnetwork will have its own 30 usable IP addresses.
Conclusion
The /27 subnet mask generally provides 32 IP address ranges per subnetwork. The first IP address in this range is reserved for network address, and the last IP address in this range is reserved for broadcast IP address. So, there are 30 usable IP address available to host devices.
The number of subnetworks obtainable when we subnet with a /27 subnet mask depends on the class of the network. As we have shown above, the class C network will yield 8 subnetworks, the class B network will yield 2048 subnetworks, and the class A network will yield 524288 subnetworks when subnetted with the /27 prefix length.
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