The /17 subnet, explained
Everything about a /17 CIDR block — mask, address count, usable hosts and where a /17 fits in real network plans.
| Prefix length | /17 — 17 network bits, 15 host bits |
| Subnet mask | 255.255.128.0 |
| Wildcard (ACL) mask | 0.0.127.255 |
| Binary mask | 11111111.11111111.10000000.00000000 |
| Total addresses | 2^15 = 32,768 |
| Usable hosts | 32,766 (total − network − broadcast) |
| Contains | 128 × /24 subnets |
| A /16 holds | 2 × /17 subnets |
| Example block | 10.10.0.0/17 |
Notes
A /17 prefix means the first 17 bits of the 32-bit IPv4 address identify the network and the remaining 15 bits identify hosts — giving 2^15 = 32,768 addresses. The first address (network) and the last (broadcast) are not assignable in a standard subnet.
Cloud sizing: a /17 is valid as an AWS VPC (allowed range /16–/28). AWS reserves 5 addresses per subnet, so a /17 subnet gives 32,763 assignable IPs there; GCP reserves 4 (32,764 assignable).
Adjacent sizes: /8 · /9 · /10 · /11 · /12 · /13 · /14 · /15 · /16 · /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 /17?
32,768 total addresses, of which 32,766 are usable hosts (network and broadcast addresses are reserved).
What is the subnet mask for /17?
255.255.128.0 (wildcard mask 0.0.127.255).
What does /17 mean in an IP address?
The /17 suffix is CIDR notation: the first 17 bits are the network part, the last 15 bits are the host part.