A /25 subnet mask is 255.255.255.128, with 128 addresses and 126 usable hosts.
| CIDR prefix | /25 |
|---|---|
| Subnet mask | 255.255.255.128 |
| Subnet mask (binary) | 11111111.11111111.11111111.10000000 |
| Wildcard mask | 0.0.0.127 |
| Network bits / host bits | 25 / 7 |
| Total addresses | 128 |
| Usable hosts | 126 |
| /25 subnets in a /8 | 131,072 |
| /25 subnets in a /16 | 512 |
| /25 subnets in a /24 | 2 |
The mask ends 1 bit into octet 4, so blocks step in increments of 128 in that octet. Half of a /24. A convenient way to split one /24 into two equal segments of 126 hosts.
| Network | First usable host | Last usable host | Broadcast |
|---|---|---|---|
| 192.168.1.0/25 | 192.168.1.1 | 192.168.1.126 | 192.168.1.127 |
| 192.168.1.128/25 | 192.168.1.129 | 192.168.1.254 | 192.168.1.255 |
A /25 subnet mask is 255.255.255.128. In binary that is 11111111.11111111.11111111.10000000: 25 network bits followed by 7 host bits. The wildcard mask is 0.0.0.127.
A /25 holds 128 addresses in total. Subtracting the network and broadcast addresses leaves 126 usable hosts.
A /24 divides into 2 subnets of size /25, because each extra mask bit doubles the number of subnets.
Calculate this over HTTP: curl https://networkcalc.com/api/ip/192.168.1.0/25.
See the Subnet Calculator API.