/23 subnet mask (255.255.254.0)

A /23 network has 9 host bits, 512 addresses and 510 usable hosts. It is 2 /24 networks.

/23 at a glance

Netmask: 255.255.254.0
Wildcard mask: 0.0.1.255
Hex mask: 0xFFFFFE00
Binary mask: 11111111.11111111.11111110.00000000
Host bits: 9
Total addresses: 512
Usable hosts: 510 (minus network and broadcast)
Compared with a /24: 2 /24 networks

Calculate a /23 in the subnet calculator

Typical uses of a /23

A /23 joins two /24s, for example 192.168.0.0 to 192.168.1.255, giving 510 usable hosts. Use it when a /24 VLAN fills up and renumbering is harder than widening the mask.

Example /23 ranges

Network	Network address	First usable	Last usable	Broadcast	Next /23
10.0.0.0/23	10.0.0.0	10.0.0.1	10.0.1.254	10.0.1.255	10.0.2.0/23	calculate
172.16.0.0/23	172.16.0.0	172.16.0.1	172.16.1.254	172.16.1.255	172.16.2.0/23	calculate
192.168.0.0/23	192.168.0.0	192.168.0.1	192.168.1.254	192.168.1.255	192.168.2.0/23	calculate

The /23 boundary falls in the third octet, so /23 networks start every 2 in that octet (128 per /16): 0, 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 30, ....

How many /23s fit

In a	Number of /23 subnets
/8	32,768
/16	128
/20	8

What a /23 contains

Smaller block	How many
/24	2
/26	8
/28	32
/29	64
/30	128
/32	512

Splitting a /23

Into	Subnets	Usable hosts each
/24	2	254
/25	4	126
/26	8	62
/27	16	30

/23 in AWS, Azure and Google Cloud

Platform	Allowed subnet sizes	/23 allowed?	Usable after reservation
AWS VPC	/16 to /28	Yes	507 (5 reserved)
Azure VNet	/2 to /29	Yes	507 (5 reserved)
Google Cloud VPC	/4 to /29	Yes	508 (4 reserved)

Cloud providers reserve addresses in every subnet (first four and the last address (network, VPC router, DNS, future use, broadcast) on AWS), so the usable count there differs from the classic 2^n - 2.