The 255.255.128.0 subnet mask.

The /17 subnet uses 255.255.128.0 as its subnet mask — meaning the first 17 bits of every address identify the network, and the remaining 15 bits identify the host within that network. That gives you 32,768 total addresses (32,766 usable on standard RFC math, after subtracting the network and broadcast addresses).

The wildcard mask — the bitwise inverse of the subnet mask — is 0.0.127.255. 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 /17, that leaves 15 don't-care host bits.

To find the network address for any IP in a /17 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 /17 holds 32,768 addresses. Useful when a /16 is more than you need and a /18 is too small. You see /17s as building or campus aggregates in large enterprises and as parent blocks for VLSM designs.

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 /17 on standard RFC math gives you 32,766 usable hosts, but on AWS or Azure that drops to 32,763. 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.