/23 Subnet Mask — 255.255.254.0

The number after the slash counts the network bits, so a /23 has 23 network bits and 9 host bits. Those 9 bits are what produce the address count: 2^9 = 512 addresses in the block.

Prefix        /23
Subnet mask   255.255.254.0
Wildcard      0.0.1.255
Binary mask   11111111.11111111.11111110.00000000
Host bits     9

Two /24s — often an aggregation of two VLANs.

Taking 192.168.0.0 as the starting point and applying the /23 mask gives the boundaries below. The network address is the base with every host bit cleared, and the broadcast address is the base with every host bit set — which is why they are always the first and last addresses in the block.

To decide whether a /23 fits a requirement, compare the usable host count against the number of devices you need to address, then add spare capacity.

Splitting a /23 by borrowing one more bit produces two /24 blocks, each half the size. Borrowing two bits produces four /25 blocks, and so on. The counts are always powers of two.

Use the VLSM planner when the subnets inside a /23 need to be different sizes — for example a /25 for a user VLAN and several /30s for the links between switches.

• Forgetting the two reserved addresses. A /23 has 512 addresses but only 510 assignable to devices. Sizing a subnet at exactly 510 hosts of requirement leaves nothing for the network and broadcast address.
• Writing the mask wrong. There is exactly one valid mask for a /23: 255.255.254.0. Anything else is a different prefix and will silently create a different network.
• Using the wildcard as the mask. 0.0.1.255 is the wildcard for a /23, not the mask. In a Cisco ACL the wildcard convention is inverted, so the two get swapped constantly.
• Starting the subnet off its boundary. A /23 must start on a multiple of 512. Starting anywhere else overlaps neighbouring subnets and produces routing loops or unreachable ranges.
• Assuming a /23 is the right size for every case. Check the host requirement first — the CIDR calculator has a host-to-prefix cheat sheet, and the subnet calculator shows the full breakdown for any address.