/23 Subnet Mask ReferenceIPv4
Two /24s walking in lockstep: mask <code>255.255.254.0</code>, 512 addresses, 510 usable. The /23 is the classic upgrade path when a thriving /24 DHCP scope presses against its 254-host ceiling.
/23 facts at a glance
- Subnet mask
- 255.255.254.0
- Wildcard mask
- 0.0.1.255
- Binary mask
- 11111111.11111111.11111110.00000000
- Total addresses
- 512
- Usable hosts
- 510 (excludes network & broadcast)
- Block size
- 512 addresses (29)
- Typical role
- Mid-size networks and ISP allocations
Example /23 networks
Three consecutive /23 blocks, starting at the /23 boundary that contains 198.51.100.0:
| Network | Usable range | Usable hosts |
|---|---|---|
| 198.51.100.0/23 | 198.51.100.1 – 198.51.101.254 | 510 |
| 198.51.102.0/23 | 198.51.102.1 – 198.51.103.254 | 510 |
| 198.51.104.0/23 | 198.51.104.1 – 198.51.105.254 | 510 |
How to calculate a /23 mask
A /23 mask sets the first 23 bits of a 32-bit number to 1 and leaves the remaining 9 bits as 0. In binary that reads 11111111.11111111.11111110.00000000, which converts to dotted decimal 255.255.254.0. Each block covers a contiguous run of 29 = 512 addresses, and every block starts at a multiple of that size. The wildcard simply flips every bit, giving 0.0.1.255.
Where /23 is used
- Growing DHCP scopes that outgrew a single /24 overnight.
- Paired server and client ranges kept adjacent for tidy ACLs.
- Compact public allocations bridging one /24 and the next.
Odd-even rule: the network's third octet must be even. <code>10.2.4.0/23</code> covers 10.2.4.0 through 10.2.5.255, while <code>10.2.5.0/23</code> is misaligned and will be rejected by strict tooling.
Frequently asked questions
How many hosts are in a /23?
510 usable of 512 total addresses.
What is the mask for a /23?
<code>255.255.254.0</code>, wildcard <code>0.0.1.255</code>; the boundary falls on the last bit of the third octet.
Why must the third octet be even in a /23?
Because the final fixed bit is the low bit of that octet, each /23 spans one even value plus the odd value immediately after it.
Open this block in a calculator
Open the /23 block in the subnet calculator