# /17 Subnet Mask (255.255.128.0) – Hosts, Range & FAQ

> The /17 subnet mask is 255.255.128.0. A /17 block holds 32,768 addresses with 32,766 usable hosts — ranges, wildcard, binary math and FAQs.

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# /17 Subnet Mask ReferenceIPv4

Half a /16 with room to breathe: mask 255.255.128.0, 32,768 addresses. The /17 fits a large site that has outgrown octet-round numbers but does not need a full /16.

The /17 subnet mask is 255.255.128.0, equivalent to 0.0.127.255 as a wildcard. A /17 block contains 32,768 IP addresses (32,766 usable hosts).

## /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: 192.168.0.0/17 runs from 192.168.0.0 to 192.168.127.255, and its sibling 192.168.128.0/17 completes the parent /16.

## Frequently asked questions

**What is the subnet mask for a /17?**

255.255.128.0, wildcard 0.0.127.255. 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](https://itinsighthub.com/subnet-calculator/?q=198.51.0.0%2F17)

[Get the /17 wildcard mask →](https://itinsighthub.com/wildcard-mask-calculator/?q=%2F17)

### Related prefixes

[/16](https://itinsighthub.com/subnet/16/)[/18](https://itinsighthub.com/subnet/18/)[/24](https://itinsighthub.com/subnet/24/)[/26](https://itinsighthub.com/subnet/26/)[/28](https://itinsighthub.com/subnet/28/)[/30](https://itinsighthub.com/subnet/30/)

← /16 subnet mask
/18 subnet mask →

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