Guide
How many hosts are in an IPv4 subnet?
Calculate total and usable IPv4 addresses from a CIDR prefix, including the correct treatment of /31 and /32.
by Tools in a Tab · Published on · Reviewed on
Short answer
An IPv4 address contains 32 bits. If a prefix reserves p bits for the network,
32 - p bits remain variable and the block contains:
total addresses = 2^(32 - p)For /27, five bits remain: 2^5 = 32 addresses.
Quick table
| Prefix | Total | Usable in a conventional subnet |
|---|---|---|
/24 |
256 | 254 |
/27 |
32 | 30 |
/28 |
16 | 14 |
/29 |
8 | 6 |
/30 |
4 | 2 |
/31 |
2 | 2 on a point-to-point link |
/32 |
1 | 1 host address or route |
A conventional subnet reserves its first address as the network identifier and
its last as the directed broadcast, so two are usually subtracted. Do not apply
that rule mechanically to /31 and /32.
Special cases
RFC 3021 permits both addresses
in a /31 on a point-to-point link: there are only two endpoints, so a separate
directed broadcast is unnecessary. A /32 identifies one address and commonly
appears as a host route.
The IPv4 calculator reports these cases explicitly rather than returning zero hosts from an out-of-context subtraction.
Capacity is not availability
The mathematical block size does not guarantee every address is assignable in
every network. Gateways, provider reservations, infrastructure, and local
policy reduce practical capacity. A /0 should not be read as billions of
usable hosts on one LAN.
For planning, calculate the block first and then document design reservations.
If you need at least 25 conventional hosts, /27 provides 30; /28 only 14.