The /25 prefix: 128 addresses, half of a /24
A /25 splits the last octet in two: 7 host bits, 128 addresses, mask 255.255.255.128. The /24 you already know becomes a pair of /25s — 192.168.1.0/25 covers .0 through .127, and 192.168.1.128/25 covers .128 through .255.
It is the first step into real subnetting, where the mask stops aligning with the dots and people start making off-by-one errors. It is also a genuinely useful production size: 126 usable hosts covers most departmental VLANs with room to spare.
| Prefix length | /25 |
| Subnet mask | 255.255.255.128 |
| Wildcard mask | 0.0.0.127 |
| Total addresses | 128 |
| Usable hosts | 126 |
Splitting a /24 the clean way
The classic reason to reach for /25 is isolation without new address space: servers on the lower half, clients on the upper half, or a voice VLAN kept apart from data. You pay a small tax — each half loses its own network and broadcast addresses, so two /25s give you 252 usable addresses where the unsplit /24 had 254.
Reading /25 ranges at a glance
There is only one trick to learn: a /25 boundary falls at .0 or .128. If someone says the network is 10.4.9.128/25, the usable range is .129 through .254 and the broadcast is .255. Get comfortable spotting which half an address lives in, and /26s and /27s will feel like the same pattern with more steps.
CIDR Calculator
Run the math on 10.0.0.0/25 — or any other block
Frequently asked questions
How many IP addresses are in a /25?
128 in total (2^7), of which 126 are usable for hosts.
What is the subnet mask of a /25?
255.255.255.128. Only the highest bit of the last octet is a network bit, which is why every /25 starts at either .0 or .128.
How do you split a /24 into two /25s?
Lower half: 192.168.1.0/25 (hosts .1–.126). Upper half: 192.168.1.128/25 (hosts .129–.254). Each /25 spends one address on the network and one on broadcast.
Is a /25 big enough for an office VLAN?
For most departments, yes — 126 usable addresses handles a floor of workstations, phones, and printers. It is the most common small production VLAN size in enterprise networks.