/25 Subnet Mask ReferenceIPv4
Slice a /24 in half and you get two /25s, each with mask <code>255.255.255.128</code> and 126 usable hosts. It is the first stop when one LAN gets crowded and you want production up front and guests in the back without changing upstream gear.
/25 facts at a glance
- Subnet mask
- 255.255.255.128
- Wildcard mask
- 0.0.0.127
- Binary mask
- 11111111.11111111.11111111.10000000
- Total addresses
- 128
- Usable hosts
- 126 (excludes network & broadcast)
- Block size
- 128 addresses (27)
- Typical role
- Subnets, VLANs and WAN links
Example /25 networks
Three consecutive /25 blocks, starting at the /25 boundary that contains 198.51.100.0:
| Network | Usable range | Usable hosts |
|---|---|---|
| 198.51.100.0/25 | 198.51.100.1 – 198.51.100.126 | 126 |
| 198.51.100.128/25 | 198.51.100.129 – 198.51.100.254 | 126 |
| 198.51.101.0/25 | 198.51.101.1 – 198.51.101.126 | 126 |
How to calculate a /25 mask
A /25 mask sets the first 25 bits of a 32-bit number to 1 and leaves the remaining 7 bits as 0. In binary that reads 11111111.11111111.11111111.10000000, which converts to dotted decimal 255.255.255.128. Each block covers a contiguous run of 27 = 128 addresses, and every block starts at a multiple of that size. The wildcard simply flips every bit, giving 0.0.0.127.
Where /25 is used
- Splitting a crowded <code>192.168.1.0/24</code> into two DHCP scopes on the same router.
- Floor-by-floor separation of workstations from printers and IoT devices.
- Small office plans that need two security zones from a single inherited block.
The upper half starts at .128: <code>192.168.1.0/25</code> serves .1 through .126, while <code>192.168.1.128/25</code> serves .129 through .254. Each half keeps its own broadcast (.127 and .255).
Ancient gear sometimes mishandled masks that broke octet boundaries; anything built this century handles a /25 without complaint.
Frequently asked questions
How many usable hosts are in a /25?
126 of 128 total addresses, after removing the network and broadcast slots.
What is the subnet mask for a /25?
<code>255.255.255.128</code>, wildcard <code>0.0.0.127</code>: seven host bits, half of a classic /24.
How do I split a /24 into two /25s?
Declare <code>x.x.x.0/25</code> for the lower range and <code>x.x.x.128/25</code> for the upper. Together they reassemble the original /24 exactly.
What is the broadcast address of the lower /25?
<code>x.x.x.127</code>, since the low bit of the fourth octet is the first fixed bit in the mask.
Open this block in a calculator
Open the /25 block in the subnet calculator