/17 · 255.255.128.0
Open in the calculator
Half a /16. A common size for a VPC that should never grow past 32,768 addresses, or for splitting a /16 between two availability zones.
Key numbers
| Prefix length | /17 (17 network bits, 15 host bits) |
| Subnet mask | 255.255.128.0 |
| Mask in binary | 11111111.11111111.10000000.00000000 |
| Wildcard mask | 0.0.127.255 |
| Total addresses | 32,768 |
| Usable hosts | 32,766 (total minus network and broadcast) |
| Contains | 128 × /24 networks |
| Block size | Increments of 128 in octet 3 |
Example ranges
Private (RFC 1918) blocks of this size. Click one to open it in the calculator.
| Block | First address | Last address | Usable hosts |
|---|---|---|---|
| 10.0.0.0/17 | 10.0.0.0 | 10.0.127.255 | 10.0.0.1 – 10.0.127.254 |
| 172.16.0.0/17 | 172.16.0.0 | 172.16.127.255 | 172.16.0.1 – 172.16.127.254 |
| 192.168.0.0/17 | 192.168.0.0 | 192.168.127.255 | 192.168.0.1 – 192.168.127.254 |
Usable addresses in the cloud
Cloud providers reserve a few addresses in every subnet for the router, DNS and future use, so the usable count is lower than the formula gives.
| Provider | Usable in a /17 | Notes |
|---|---|---|
| AWS VPC subnet | 32,763 | Allowed (/16 to /28). AWS reserves the first four addresses and the last one. |
| Azure virtual network subnet | 32,763 | Allowed. Azure reserves the first four addresses and the last one. |
| Google Cloud subnet | 32,764 | Allowed. Google Cloud reserves the first two and the last two addresses. |
How the numbers are derived
An IPv4 address has 32 bits. A /17 uses 17 of them for the network, leaving 15 host bits, so the block has 215 = 32,768 addresses. The first address (all host bits 0) is the network address and the last (all host bits 1) is the broadcast address, which leaves 32,766 for hosts. The subnet mask is 17 ones followed by 15 zeros, written in dotted decimal as 255.255.128.0; the wildcard mask 0.0.127.255 is its inverse, used in Cisco ACLs and OSPF configuration.
FAQ
How many hosts can a /17 have?
32,768 addresses in total, of which 32,766 are usable after removing the network and broadcast addresses. Cloud providers reserve a few more: 32,763 usable in AWS and Azure, 32,764 in Google Cloud.
How many /24 subnets fit in a /17?
128. Each step of the prefix length halves or doubles the block, so a /17 contains 2^(24 − 17) = 128 networks of /24 size.