The /25 subnet, explained
Everything about a /25 CIDR block — mask, address count, usable hosts and where a /25 fits in real network plans.
| Prefix length | /25 — 25 network bits, 7 host bits |
| Subnet mask | 255.255.255.128 |
| Wildcard (ACL) mask | 0.0.0.127 |
| Binary mask | 11111111.11111111.11111111.10000000 |
| Total addresses | 2^7 = 128 |
| Usable hosts | 126 (total − network − broadcast) |
| A /16 holds | 512 × /25 subnets |
| Example block | 192.168.1.0/25 |
Notes
A /25 prefix means the first 25 bits of the 32-bit IPv4 address identify the network and the remaining 7 bits identify hosts — giving 2^7 = 128 addresses. The first address (network) and the last (broadcast) are not assignable in a standard subnet.
Cloud sizing: a /25 is valid as an AWS VPC (allowed range /16–/28). AWS reserves 5 addresses per subnet, so a /25 subnet gives 123 assignable IPs there; GCP reserves 4 (124 assignable).
Adjacent sizes: /8 · /9 · /10 · /11 · /12 · /13 · /14 · /15 · /16 · /17 · /18 · /19 · /20 · /21 · /22 · /23 · /24 · /26 · /27 · /28 · /29 · /30 · /31 · /32
Frequently asked questions
How many IP addresses are in a /25?
128 total addresses, of which 126 are usable hosts (network and broadcast addresses are reserved).
What is the subnet mask for /25?
255.255.255.128 (wildcard mask 0.0.0.127).
What does /25 mean in an IP address?
The /25 suffix is CIDR notation: the first 25 bits are the network part, the last 7 bits are the host part.