/23 Subnet Mask

A /23 subnet mask is 255.255.254.0. It has 512 addresses, 510 usable hosts and the wildcard mask 0.0.1.255.

CIDR prefix/23
Subnet mask255.255.254.0
Wildcard mask0.0.1.255
Binary mask11111111.11111111.11111110.00000000
Total addresses512
Usable hosts510
Block size2 (third octet)

Example: 192.168.0.0/23

Network address192.168.0.0
First usable host192.168.0.1
Last usable host192.168.1.254
Broadcast address192.168.1.255

Where /23 subnets start

/23 networks start on multiples of 2 in the third octet (0, 2, 4, 6, …). A /16 holds 128 /23 subnets.

How to calculate a /23 subnet

  1. Host bits: 32 − 23 = 9.
  2. Total addresses: 29 = 512.
  3. Usable hosts: 512 − 2 = 510, because the network and broadcast addresses can't be assigned to hosts.
  4. Subnet mask: 23 one-bits followed by 9 zero-bits. The third octet is 254 (256 − 2), which gives 255.255.254.0.
  5. Wildcard mask: 255.255.255.255 − 255.255.254.0 = 0.0.1.255.

When to use a /23

A /23 combines two /24 networks. For example, 192.168.0.0/23 covers 192.168.0.0 to 192.168.1.255.

Updated . Calculate any network with the subnet calculator, or compare all prefixes in the subnet cheat sheet.