Last reviewed: May 2026
A /23 subnet uses 23 bits for the network portion of an IPv4 address, leaving 9 bits for host addressing. This produces 512 total IP addresses and 510 usable hosts. A /23 spans exactly two consecutive /24 blocks — useful when one /24 (254 hosts) is too small but a /22 (1,022 hosts) is too large.
Calculate any /23 subnet instantly →| Property | Value |
|---|---|
| Prefix length | /23 |
| Subnet mask | 255.255.254.0 |
| Wildcard mask | 0.0.1.255 |
| Total addresses | 512 |
| Usable host addresses | 510 |
| Network address | x.x.x.0 (third octet must be even: .0, .2, .4, .6, …) |
| Broadcast address | x.x.1.255 (spans two third-octet values) |
| Binary mask | 11111111.11111111.11111110.00000000 |
| AWS usable hosts | 507 (AWS reserves 5 IPs per subnet) |
Unlike a /24 (which stays within one third-octet value), a /23 covers two consecutive values. For example, 10.0.0.0/23 includes addresses from 10.0.0.0 all the way through 10.0.1.255. The third octet of the network address must always be even — you cannot have a /23 starting at 10.0.1.0 (odd third octet).
| Subnet | Range | Spans /24s | AWS Usable |
|---|---|---|---|
| 10.0.0.0/23 | 10.0.0.0 – 10.0.1.255 | 10.0.0.0/24 + 10.0.1.0/24 | 507 |
| 10.0.2.0/23 | 10.0.2.0 – 10.0.3.255 | 10.0.2.0/24 + 10.0.3.0/24 | 507 |
| 10.0.4.0/23 | 10.0.4.0 – 10.0.5.255 | 10.0.4.0/24 + 10.0.5.0/24 | 507 |
| 10.0.6.0/23 | 10.0.6.0 – 10.0.7.255 | 10.0.6.0/24 + 10.0.7.0/24 | 507 |
| Parent Block | Number of /23s |
|---|---|
| /16 | 128 |
| /22 | 2 |
| /24 | 0.5 (a /23 is 2× larger than a /24) |
| CIDR | Subnet Mask | Total IPs | Usable Hosts | AWS Usable | Typical Use |
|---|---|---|---|---|---|
| /22 | 255.255.252.0 | 1,024 | 1,022 | 1,019 | Large office VLAN, Azure VM subnets |
| /23 | 255.255.254.0 | 512 | 510 | 507 | Medium office; 2×/24 merged |
| /24 | 255.255.255.0 | 256 | 254 | 251 | Standard workload subnet |
| /25 | 255.255.255.128 | 128 | 126 | 121 | Half a /24; two-zone segmentation |
A /23 subnet has 510 usable host addresses from a total of 512 IPs. Two addresses are reserved: the network address and the broadcast address. In AWS, 507 IPs are usable after the 5 reserved addresses are subtracted.
A /23 spans exactly 2 consecutive /24 blocks. For example, 10.0.0.0/23 covers both 10.0.0.0/24 and 10.0.1.0/24. The third octet of a /23 network address must be an even number (0, 2, 4, 6, 8, etc.).
The subnet mask for a /23 network is 255.255.254.0. In binary, 23 bits are set to 1 followed by 9 bits of 0: 11111111.11111111.11111110.00000000.