# /23 Subnet Mask (255.255.254.0) – Hosts, Range & FAQ > The /23 subnet mask is 255.255.254.0. A /23 block holds 512 addresses with 510 usable hosts — ranges, wildcard, binary math and FAQs. - Page URL: https://itinsighthub.com/subnet/23/ - Markdown variant of this page: append `?format=md` to any URL on this site or send `Accept: text/markdown`. - Full site index for AI assistants: https://itinsighthub.com/llms.txt - Full site export: https://itinsighthub.com/llms-full.txt # /23 Subnet Mask ReferenceIPv4 Two /24s walking in lockstep: mask 255.255.254.0, 512 addresses, 510 usable. The /23 is the classic upgrade path when a thriving /24 DHCP scope presses against its 254-host ceiling. The /23 subnet mask is 255.255.254.0, equivalent to 0.0.1.255 as a wildcard. A /23 block contains 512 IP addresses (510 usable hosts). ## /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. 10.2.4.0/23 covers 10.2.4.0 through 10.2.5.255, while 10.2.5.0/23 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?** 255.255.254.0, wildcard 0.0.1.255; 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](https://itinsighthub.com/subnet-calculator/?q=198.51.100.0%2F23) [Get the /23 wildcard mask →](https://itinsighthub.com/wildcard-mask-calculator/?q=%2F23) ### Related prefixes [/22](https://itinsighthub.com/subnet/22/)[/24](https://itinsighthub.com/subnet/24/)[/26](https://itinsighthub.com/subnet/26/)[/28](https://itinsighthub.com/subnet/28/)[/30](https://itinsighthub.com/subnet/30/) ← /22 subnet mask /24 subnet mask → --- © 2026 ITInsightHub · [About](https://itinsighthub.com/about/) · [Contact](https://itinsighthub.com/contact/) · [Privacy](https://itinsighthub.com/privacy/)