The /27 subnet, explained
Everything about a /27 CIDR block — mask, address count, usable hosts and where a /27 fits in real network plans.
| Prefix length | /27 — 27 network bits, 5 host bits |
| Subnet mask | 255.255.255.224 |
| Wildcard (ACL) mask | 0.0.0.31 |
| Binary mask | 11111111.11111111.11111111.11100000 |
| Total addresses | 2^5 = 32 |
| Usable hosts | 30 (total − network − broadcast) |
| A /16 holds | 2,048 × /27 subnets |
| Example block | 192.168.1.0/27 |
Notes
A /27 prefix means the first 27 bits of the 32-bit IPv4 address identify the network and the remaining 5 bits identify hosts — giving 2^5 = 32 addresses. The first address (network) and the last (broadcast) are not assignable in a standard subnet.
Cloud sizing: a /27 is valid as an AWS VPC (allowed range /16–/28). AWS reserves 5 addresses per subnet, so a /27 subnet gives 27 assignable IPs there; GCP reserves 4 (28 assignable).
Adjacent sizes: /8 · /9 · /10 · /11 · /12 · /13 · /14 · /15 · /16 · /17 · /18 · /19 · /20 · /21 · /22 · /23 · /24 · /25 · /26 · /28 · /29 · /30 · /31 · /32
Frequently asked questions
How many IP addresses are in a /27?
32 total addresses, of which 30 are usable hosts (network and broadcast addresses are reserved).
What is the subnet mask for /27?
255.255.255.224 (wildcard mask 0.0.0.31).
What does /27 mean in an IP address?
The /27 suffix is CIDR notation: the first 27 bits are the network part, the last 5 bits are the host part.