The /27 prefix means the first 27 bits of the address identify the network and the remaining 5 identify the host. Written out, the mask is 255.255.255.224, in binary 11111111.11111111.11111111.11100000, and its Cisco wildcard or inverse form is 0.0.0.31. It covers 32 addresses, of which 30 can be given to hosts.
| CIDR prefix | /27 |
|---|---|
| Subnet mask | 255.255.255.224 |
| Mask in binary | 11111111.11111111.11111111.11100000 |
| Wildcard (inverse) mask | 0.0.0.31 |
| Host bits | 5 |
| Total addresses | 32 |
| Usable hosts | 30 |
| Block size (magic number) | 32 in octet 4 |
A 192.168.1.0/24 splits into exactly 8 subnets of this size.
| Network | First host | Last host | Broadcast |
|---|---|---|---|
| 192.168.1.0/27 | 192.168.1.1 | 192.168.1.30 | 192.168.1.31 |
| 192.168.1.32/27 | 192.168.1.33 | 192.168.1.62 | 192.168.1.63 |
| 192.168.1.64/27 | 192.168.1.65 | 192.168.1.94 | 192.168.1.95 |
| 192.168.1.96/27 | 192.168.1.97 | 192.168.1.126 | 192.168.1.127 |
| 192.168.1.128/27 | 192.168.1.129 | 192.168.1.158 | 192.168.1.159 |
| 192.168.1.160/27 | 192.168.1.161 | 192.168.1.190 | 192.168.1.191 |
| 192.168.1.192/27 | 192.168.1.193 | 192.168.1.222 | 192.168.1.223 |
| 192.168.1.224/27 | 192.168.1.225 | 192.168.1.254 | 192.168.1.255 |
Take the interesting octet of the mask, the one that is neither 255 nor 0. For /27 that is 224 in octet 4. Subtract it from 256:
256 - 224 = 32
That is the block size. Every /27 network address is a multiple of 32 in octet 4, so you can list them by counting: 0, 32, 64, 96, and so on. No binary needed once you know the number.
A /27 gives 30 usable addresses. It is the size of a small branch office, a printer VLAN, or the out-of-band management network where every switch and PDU needs an address and nothing else does.
A /27 holds 32 addresses in total. After reserving the network and broadcast addresses, 30 are usable by hosts.
The /27 subnet mask is 255.255.255.224 in dotted-decimal notation.
The wildcard mask, also called the inverse mask, is 0.0.0.31. It is the bitwise inverse of 255.255.255.224 and is what Cisco access lists and OSPF network statements expect.
The /27 mask written out bit by bit is 11111111.11111111.11111111.11100000: 27 ones followed by 5 zeros.
A /24 splits into 8 subnets of /27.