Last reviewed: May 2026
A /25 subnet contains 128 total IP addresses and 126 usable host addresses. The "/25" prefix uses 25 bits for the network portion, leaving 7 bits (27 = 128) for hosts. Its most common use is splitting a /24 exactly in half — creating two equal segments from one standard subnet.
Calculate any /25 subnet instantly →| Property | Value |
|---|---|
| Prefix length | /25 |
| Subnet mask | 255.255.255.128 |
| Wildcard mask | 0.0.0.127 |
| Total addresses | 128 |
| Usable host addresses | 126 |
| First /25 of a /24 | x.x.x.0 – x.x.x.127 (network .0, broadcast .127) |
| Second /25 of a /24 | x.x.x.128 – x.x.x.255 (network .128, broadcast .255) |
| Binary mask | 11111111.11111111.11111111.10000000 |
| AWS usable hosts | 121 (AWS reserves 5 IPs per subnet) |
A /24 divides into exactly two /25 subnets. This is the simplest way to segment a standard network block into two zones — for example, separating servers (.0/25) from clients (.128/25), or wired from wireless devices. Each half gets 126 usable addresses.
| Subnet | Network | Usable Range | Broadcast | Usable Hosts |
|---|---|---|---|---|
| First half | 192.168.1.0/25 | 192.168.1.1 – 192.168.1.126 | 192.168.1.127 | 126 |
| Second half | 192.168.1.128/25 | 192.168.1.129 – 192.168.1.254 | 192.168.1.255 | 126 |
| CIDR | Subnet Mask | Total IPs | Usable Hosts | AWS Usable | Typical Use |
|---|---|---|---|---|---|
| /24 | 255.255.255.0 | 256 | 254 | 251 | Standard subnet — most common |
| /25 | 255.255.255.128 | 128 | 126 | 121 | Half a /24; two-zone segmentation |
| /26 | 255.255.255.192 | 64 | 62 | 59 | Quarter of /24; Azure managed subnets |
| /27 | 255.255.255.224 | 32 | 30 | 27 | Infrastructure / gateway subnets |
A /16 block contains 2(25−16) = 512 subnets of /25 size. A /24 contains exactly 2.
Split a /24 into two /25s with SubnetSolver →