/23 Subnet: Mask 255.255.254.0

How many /23 networks fit

A /23 holds 2 × /24. Two /23s make a /22.

Worked example: 192.168.0.255/23

A home lab widened its network to a /23 and someone wants to give a server 192.168.0.255. In a /24 that would be a broadcast address. Is it one here?

1. The mask is 255.255.254.0. The third octet (254) is the "interesting" one — the first that is not 255.
2. Block size = 256 − 254 = 2. Networks start at multiples of 2 in the third octet: 0, 2, 4, …
3. The third octet of 192.168.0.255 is 0. 0 ÷ 2 = 0 remainder 0, so the block starts at 0 × 2 = 0.
4. Network = 192.168.0.0 (octets after the third become 0). Broadcast = 0 + 2 − 1 = 1, giving 192.168.1.255 (octets after it become 255).
5. Usable hosts: 192.168.0.1 to 192.168.1.254, which is 2⁹ − 2 = 510 addresses.

Address  11000000.10101000.0000000|0.11111111
Mask     11111111.11111111.1111111|0.00000000
Network  11000000.10101000.0000000|0.00000000

Where /23 is used in real networks

• A /24 that ran out

  Offices with 300–500 devices often use a /23 for one user VLAN. You keep one subnet, one gateway and one DHCP scope.

• Home routers with many smart devices

  Some homes with lots of IoT gear switch from 192.168.0.0/24 to 192.168.0.0/23 to get 510 addresses without adding VLANs.

Common /23 mistakes

• Writing 192.168.1.0/23. A /23 must start on an even third octet, so it is really 192.168.0.0/23 — covering 192.168.0.0 to 192.168.1.255.
• Refusing to assign 192.168.0.255 or 192.168.1.0 in a /23. They are in the middle of the block and are normal host addresses. Only 192.168.0.0 and 192.168.1.255 are reserved.
• Assuming every device copes with middle .0 or .255 addresses. A few old IoT devices and printers reject them. If one does, give it a different address rather than shrinking the subnet.

/23 FAQ

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