The /9 subnet, explained
Everything about a /9 CIDR block — mask, address count, usable hosts and where a /9 fits in real network plans.
| Prefix length | /9 — 9 network bits, 23 host bits |
| Subnet mask | 255.128.0.0 |
| Wildcard (ACL) mask | 0.127.255.255 |
| Binary mask | 11111111.10000000.00000000.00000000 |
| Total addresses | 2^23 = 8,388,608 |
| Usable hosts | 8,388,606 (total − network − broadcast) |
| Contains | 32,768 × /24 subnets |
| Example block | 10.0.0.0/9 |
Notes
A /9 prefix means the first 9 bits of the 32-bit IPv4 address identify the network and the remaining 23 bits identify hosts — giving 2^23 = 8,388,608 addresses. The first address (network) and the last (broadcast) are not assignable in a standard subnet.
Cloud sizing: a /9 is larger than the maximum AWS VPC (/16). Blocks this size are carved into multiple VPCs or used in on-prem/enterprise addressing plans.
Adjacent sizes: /8 · /10 · /11 · /12 · /13 · /14 · /15 · /16 · /17 · /18 · /19 · /20 · /21 · /22 · /23 · /24 · /25 · /26 · /27 · /28 · /29 · /30 · /31 · /32
Frequently asked questions
How many IP addresses are in a /9?
8,388,608 total addresses, of which 8,388,606 are usable hosts (network and broadcast addresses are reserved).
What is the subnet mask for /9?
255.128.0.0 (wildcard mask 0.127.255.255).
What does /9 mean in an IP address?
The /9 suffix is CIDR notation: the first 9 bits are the network part, the last 23 bits are the host part.