/23 vs /25 — Subnet Comparison
A /23 subnet is 4× larger than a /25. Every additional bit in the prefix halves the address space — the 2-bit difference between these two means /23 has 22 = 4 times as many addresses.
512 IPs — two /24s combined
Typical Uses
- →Two-floor office network
- →Expanded department VLAN
- →Route aggregation of two /24s
126 usable hosts — half a /24
Typical Uses
- →Public vs private half of a /24
- →Department sub-segment
- →Smaller cloud application subnets
Key Differences
How 4 /25 Subnets Divide a /23
Example using 10.0.0.0/23 as the parent block.
| # | CIDR | Network | First Usable | Last Usable | Broadcast | Hosts |
|---|---|---|---|---|---|---|
| 1 | 10.0.0.0/25 | 10.0.0.0 | 10.0.0.1 | 10.0.0.126 | 10.0.0.127 | 126 |
| 2 | 10.0.0.128/25 | 10.0.0.128 | 10.0.0.129 | 10.0.0.254 | 10.0.0.255 | 126 |
| 3 | 10.0.1.0/25 | 10.0.1.0 | 10.0.1.1 | 10.0.1.126 | 10.0.1.127 | 126 |
| 4 | 10.0.1.128/25 | 10.0.1.128 | 10.0.1.129 | 10.0.1.254 | 10.0.1.255 | 126 |
FAQ
What is the difference between /23 and /25?
A /23 has 510 usable hosts
and a /25 has 126.
The subnet masks differ: /23 uses 255.255.254.0
while /25 uses 255.255.255.128.
Every additional bit in the prefix halves the number of addresses — so the 2-bit gap means
/23 is exactly 4× larger.
How many /25 subnets fit in a /23?
Exactly 4 /25 subnets fit perfectly inside one /23 with no wasted space. To split a /23 into /25s, just increment the last 2 bits of the network address for each new subnet.
Which should I choose?
/23 is typically used for: Aggregated /24 pair. /25 is better for: Half a /24 — public/private split. Choose the smallest prefix that comfortably fits your host count — over-allocating wastes address space, but under-allocating means painful renumbering later.