/25 · 255.255.255.128
Open in the calculator
Half a /24: 128 addresses, 126 usable. Splits a /24 into .0–.127 and .128–.255. Useful for two small subnets, for example web and database tiers, inside one /24.
Key numbers
| Prefix length | /25 (25 network bits, 7 host bits) |
| Subnet mask | 255.255.255.128 |
| Mask in binary | 11111111.11111111.11111111.10000000 |
| Wildcard mask | 0.0.0.127 |
| Total addresses | 128 |
| Usable hosts | 126 (total minus network and broadcast) |
| Fits in a /24 | 2 times |
| Block size | Increments of 128 in octet 4 |
Example ranges
Private (RFC 1918) blocks of this size. Click one to open it in the calculator.
| Block | First address | Last address | Usable hosts |
|---|---|---|---|
| 10.0.0.0/25 | 10.0.0.0 | 10.0.0.127 | 10.0.0.1 – 10.0.0.126 |
| 172.16.0.0/25 | 172.16.0.0 | 172.16.0.127 | 172.16.0.1 – 172.16.0.126 |
| 192.168.0.0/25 | 192.168.0.0 | 192.168.0.127 | 192.168.0.1 – 192.168.0.126 |
Usable addresses in the cloud
Cloud providers reserve a few addresses in every subnet for the router, DNS and future use, so the usable count is lower than the formula gives.
| Provider | Usable in a /25 | Notes |
|---|---|---|
| AWS VPC subnet | 123 | Allowed (/16 to /28). AWS reserves the first four addresses and the last one. |
| Azure virtual network subnet | 123 | Allowed. Azure reserves the first four addresses and the last one. |
| Google Cloud subnet | 124 | Allowed. Google Cloud reserves the first two and the last two addresses. |
How the numbers are derived
An IPv4 address has 32 bits. A /25 uses 25 of them for the network, leaving 7 host bits, so the block has 27 = 128 addresses. The first address (all host bits 0) is the network address and the last (all host bits 1) is the broadcast address, which leaves 126 for hosts. The subnet mask is 25 ones followed by 7 zeros, written in dotted decimal as 255.255.255.128; the wildcard mask 0.0.0.127 is its inverse, used in Cisco ACLs and OSPF configuration.
FAQ
How many hosts can a /25 have?
128 addresses in total, of which 126 are usable after removing the network and broadcast addresses. Cloud providers reserve a few more: 123 usable in AWS and Azure, 124 in Google Cloud.
How many /25 subnets fit in a /24?
2. A /24 has 256 addresses and a /25 has 128, so 256 ÷ 128 = 2 blocks.