/17 Subnet Mask ReferenceIPv4
Half a /16 with room to breathe: mask <code>255.255.128.0</code>, 32,768 addresses. The /17 fits a large site that has outgrown octet-round numbers but does not need a full /16.
/17 facts at a glance
- Subnet mask
- 255.255.128.0
- Wildcard mask
- 0.0.127.255
- Binary mask
- 11111111.11111111.10000000.00000000
- Total addresses
- 32,768
- Usable hosts
- 32,766 (excludes network & broadcast)
- Block size
- 32,768 addresses (215)
- Typical role
- Mid-size networks and ISP allocations
Example /17 networks
Three consecutive /17 blocks, starting at the /17 boundary that contains 198.51.100.0:
| Network | Usable range | Usable hosts |
|---|---|---|
| 198.51.0.0/17 | 198.51.0.1 – 198.51.127.254 | 32,766 |
| 198.51.128.0/17 | 198.51.128.1 – 198.51.255.254 | 32,766 |
| 198.52.0.0/17 | 198.52.0.1 – 198.52.127.254 | 32,766 |
How to calculate a /17 mask
A /17 mask sets the first 17 bits of a 32-bit number to 1 and leaves the remaining 15 bits as 0. In binary that reads 11111111.11111111.10000000.00000000, which converts to dotted decimal 255.255.128.0. Each block covers a contiguous run of 215 = 32,768 addresses, and every block starts at a multiple of that size. The wildcard simply flips every bit, giving 0.0.127.255.
Where /17 is used
- Large-office segments carved out of a corporate /16.
- DHCP scopes sized for thousands of clients with growth headroom.
- Aggregating 128 /24s into one summary during readdressing projects.
Example: <code>192.168.0.0/17</code> runs from 192.168.0.0 to 192.168.127.255, and its sibling <code>192.168.128.0/17</code> completes the parent /16.
Frequently asked questions
What is the subnet mask for a /17?
<code>255.255.128.0</code>, wildcard <code>0.0.127.255</code>. The seventeenth bit is the top bit of the third octet.
How many addresses are in a /17?
32,768 total, with 32,766 usable under the usual convention.
How many /24s are in a /17?
128: either the lower half of a /16 or the upper half, depending on which /17 you took.
Open this block in a calculator
Open the /17 block in the subnet calculator