/27 subnet.
A /27 is a small IPv4 block containing 32 total addresses. It uses 27 network bits and 5 host bits, with subnet mask 255.255.255.224.
What does /27 mean?
IPv4 addresses contain 32 bits. A /27 CIDR prefix means the first 27 bits identify the network and the remaining 5 bits identify addresses within that network. Because there are 5 host bits, the block contains 25, or 32, total IPv4 addresses. In practical terms, /27 is a small-to-medium subnet often used for segmented LANs or infrastructure.
/27 subnet mask
The dotted-decimal subnet mask for /27 is 255.255.255.224. Its inverse wildcard mask is 0.0.0.31. These are different notations for the same network/host boundary: the subnet mask marks network bits with ones, while the wildcard mask flips those bits.
How many hosts are in a /27?
Under conventional IPv4 subnetting, a /27 has 30 usable host addresses after reserving the network and broadcast addresses.
How /27 boundaries behave
Within a /24-style final-octet view, /27 blocks advance in increments of 32 addresses when the prefix is at least /24. This makes network boundaries predictable and easy to verify. Increasing the prefix to /28 halves the addresses in each child block. Decreasing it to /26 combines this address space with an adjacent block of the same size.
When would you use a /27?
The right prefix depends on route aggregation, host requirements, topology and operational policy rather than on a single universal rule. Large prefixes with smaller numbers represent broad address aggregates; prefixes closer to /32 represent progressively smaller blocks. For ordinary IPv4 LAN planning, always verify that the chosen prefix has enough conventional usable host addresses and leaves appropriate room for growth.
Compare nearby CIDR prefixes
Frequently asked questions
What subnet mask is /27?
The IPv4 subnet mask for /27 is 255.255.255.224.
How many IP addresses are in a /27?
A /27 contains 32 total IPv4 addresses.
How many usable hosts are in a /27?
Under conventional IPv4 subnetting, /27 provides 30 usable host addresses after reserving network and broadcast.
Use the interactive calculator below with a real IPv4 address, or open the complete subnetting cheat sheet to compare /27 with every other CIDR size.