The /23 prefix: 512 addresses, two /24s merged
A /23 is 512 addresses: 9 host bits, mask 255.255.254.0. It is the first size larger than a /24 — the third octet moves in pairs, so 10.1.0.0/23 runs from 10.1.0.0 to 10.1.1.255, fusing two adjacent /24s into a single subnet.
You meet /23s in two places: growing LANs that doubled their /24, and BGP tables, because /23 sits just inside the line of what the public Internet will carry.
| Prefix length | /23 |
| Subnet mask | 255.255.254.0 |
| Wildcard mask | 0.0.1.255 |
| Total addresses | 512 |
| Usable hosts | 510 |
One step inside the BGP floor
Public routing effectively bottoms out at /24: filters on most transit providers drop anything more specific. A /23 announcement is still routable everywhere, which makes it the tightest aggregation you can build out of two /24s without losing reachability. If you hold two adjacent /24s, announcing the covering /23 — with or without the specifics — is standard practice.
Merging two /24s without renumbering the world
When two adjacent /24s serve the same site, widening the mask to /23 merges them with almost no pain: hosts keep their addresses, and only the subnet mask and gateway config change. The catch is alignment — the pair must start on an even third octet (.0–.1, .2–.3). 192.168.1.0/24 and 192.168.2.0/24 can never merge into a /23; the boundary does not fall there.
CIDR Calculator
Run the math on 10.0.0.0/23 — or any other block
Frequently asked questions
How many IP addresses are in a /23?
512 in total (2^9), with 510 usable for hosts.
What is the subnet mask of a /23?
255.255.254.0. The third octet contributes seven network bits, so /23 networks begin on even numbers in that octet.
Can a /23 be announced on the public Internet?
Yes. The de facto BGP maximum specificity is /24, so a /23 routes fine everywhere. It is the usual way to aggregate two adjacent /24 allocations.
Which pairs of /24s can combine into a /23?
Only aligned pairs: .0/.1, .2/.3, .4/.5 and so on. The lower network address must have an even third octet, otherwise no single /23 covers both.