The 255.255.254.0 subnet mask.

The /23 subnet uses 255.255.254.0 as its subnet mask — meaning the first 23 bits of every address identify the network, and the remaining 9 bits identify the host within that network. That gives you 512 total addresses (510 usable on standard RFC math, after subtracting the network and broadcast addresses).

The wildcard mask — the bitwise inverse of the subnet mask — is 0.0.1.255. Wildcards are what Cisco access-control lists and OSPF area definitions use instead of subnet masks; the "1" bits mark "don't care" positions. For a /23, that leaves 9 don't-care host bits.

To find the network address for any IP in a /23 block, perform a bitwise AND between the IP and the subnet mask. To find the broadcast, OR the network address with the wildcard. Modern tools — like our subnet calculator — do this in microseconds, but the underlying mechanics are straightforward binary arithmetic.

A /23 holds 512 addresses. Doubles a /24's host capacity, useful when a single /24 fills up but you don't want to renumber. Common in user-VLAN expansions and Wi-Fi guest networks at scale.

Cloud-provider quirks matter at every prefix size: AWS and Azure reserve 5 IPs per subnet, GCP reserves 4, and OCI reserves 3. So a /23 on standard RFC math gives you 510 usable hosts, but on AWS or Azure that drops to 507. The capacity-planning gap bites hardest at small prefixes (a /28 has 14 usable on paper, only 11 on AWS) but exists at every size. Our cloud-aware calculator applies the right math automatically.