The 255.128.0.0 subnet mask.

The /9 subnet uses 255.128.0.0 as its subnet mask — meaning the first 9 bits of every address identify the network, and the remaining 23 bits identify the host within that network. That gives you 8.39M total addresses (8.39M usable on standard RFC math, after subtracting the network and broadcast addresses).

The wildcard mask — the bitwise inverse of the subnet mask — is 0.127.255.255. Wildcards are what Cisco access-control lists and OSPF area definitions use instead of subnet masks; the "1" bits mark "don't care" positions. For a /9, that leaves 23 don't-care host bits.

To find the network address for any IP in a /9 block, perform a bitwise AND between the IP and the subnet mask. To find the broadcast, OR the network address with the wildcard. Modern tools — like our subnet calculator — do this in microseconds, but the underlying mechanics are straightforward binary arithmetic.

A /9 is half of a Class A network — 8.4 million addresses. You see /9 as an aggregate route in BGP tables and as a planning unit for ISP-scale allocations. It's never a usable LAN; the broadcast domain would be absurd.

Cloud-provider quirks matter at every prefix size: AWS and Azure reserve 5 IPs per subnet, GCP reserves 4, and OCI reserves 3. So a /9 on standard RFC math gives you 8.39M usable hosts, but on AWS or Azure that drops to 8.39M. The capacity-planning gap bites hardest at small prefixes (a /28 has 14 usable on paper, only 11 on AWS) but exists at every size. Our cloud-aware calculator applies the right math automatically.