A /23 subnet uses 23 bits for the network portion of an IPv4 address, leaving 9 bits for host addressing. This produces 512 total IP addresses and 510 usable hosts. A /23 spans exactly two consecutive /24 blocks — useful when one /24 (254 hosts) is too small but a /22 (1,022 hosts) is too large.
x.x.x.0 (third octet must be even: .0, .2, .4, .6, …)
Broadcast address
x.x.1.255 (spans two third-octet values)
Binary mask
11111111.11111111.11111110.00000000
AWS usable hosts
507 (AWS reserves 5 IPs per subnet)
A /23 Spans Two Third-Octet Values
Unlike a /24 (which stays within one third-octet value), a /23 covers two consecutive values. For example, 10.0.0.0/23 includes addresses from 10.0.0.0 all the way through 10.0.1.255. The third octet of the network address must always be even — you cannot have a /23 starting at 10.0.1.0 (odd third octet).
Example /23 Ranges in 10.0.0.0/16
Subnet
Range
Spans /24s
AWS Usable
10.0.0.0/23
10.0.0.0 – 10.0.1.255
10.0.0.0/24 + 10.0.1.0/24
507
10.0.2.0/23
10.0.2.0 – 10.0.3.255
10.0.2.0/24 + 10.0.3.0/24
507
10.0.4.0/23
10.0.4.0 – 10.0.5.255
10.0.4.0/24 + 10.0.5.0/24
507
10.0.6.0/23
10.0.6.0 – 10.0.7.255
10.0.6.0/24 + 10.0.7.0/24
507
Common Use Cases for /23 Subnets
Medium office networks — 500+ workstations and devices without needing a full /22
AWS private subnets — 507 usable IPs; fits a busy EC2 Auto Scaling group with room for ELB nodes
University department LANs — a /23 per department gives each building its own broadcast domain with room to grow
BGP route summarisation — aggregate two /24s into a single more-specific announcement
SD-WAN branch sites — large branch offices that outgrow a /24 but don't need a /22