# /9 Subnet Mask (255.128.0.0) – Hosts, Range & FAQ

> The /9 subnet mask is 255.128.0.0. A /9 block holds 8,388,608 addresses with 8,388,606 usable hosts — ranges, wildcard, binary math and FAQs.

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# /9 Subnet Mask ReferenceIPv4

Slice a /8 down the middle and you hold a /9: mask 255.128.0.0, 8,388,608 addresses. It is the first mask that climbs past a clean octet boundary, which makes it the canonical example for teaching bit boundaries.

The /9 subnet mask is 255.128.0.0, equivalent to 0.127.255.255 as a wildcard. A /9 block contains 8,388,608 IP addresses (8,388,606 usable hosts).

## /9 facts at a glance

- **Subnet mask:** 255.128.0.0
- **Wildcard mask:** 0.127.255.255
- **Binary mask:** 11111111.10000000.00000000.00000000
- **Total addresses:** 8,388,608
- **Usable hosts:** 8,388,606 (excludes network & broadcast)
- **Block size:** 8,388,608 addresses (223)
- **Typical role:** Large enterprise and carrier aggregation

## Example /9 networks

A /9 spans a large share of the address space, so one aligned example says it all.

| Network | Usable range | Usable hosts |
| --- | --- | --- |
| 198.0.0.0/9 | 198.0.0.1 – 198.127.255.254 | 8,388,606 |

## How to calculate a /9 mask

A /9 mask sets the first 9 bits of a 32-bit number to 1 and leaves the remaining 23 bits as 0. In binary that reads `11111111.10000000.00000000.00000000`, which converts to dotted decimal `255.128.0.0`. Each block covers a contiguous run of 223 = 8,388,608 addresses, and every block starts at a multiple of that size. The wildcard simply flips every bit, giving `0.127.255.255`.

## Where /9 is used

- Deaggregation demonstrations: announcing a /8 as two /9s is the classic traffic-engineering move.
- Internal region sizing for operators dividing an inherited /8.
- Practice reading masks whose ones spill into the second octet.

Landmark example: 10.128.0.0/9 is precisely the upper half of 10.0.0.0/8, spanning 10.128.0.0 through 10.255.255.255, with 10.0.0.0/9 as the lower half.

## Frequently asked questions

**What is the subnet mask for a /9?**

255.128.0.0, wildcard 0.127.255.255. The ninth bit is the highest bit of the second octet.

**How many addresses are in a /9?**

8,388,608 total, of which 8,388,606 are usable under the usual convention.

**How do you split a /8 into two /9s?**

Keep the lower half as x.0.0.0/9 and promote the upper half to x.128.0.0/9. Together they cover the original /8 with no gaps or overlaps.

## Open this block in a calculator

[Open the /9 block in the subnet calculator](https://itinsighthub.com/subnet-calculator/?q=198.0.0.0%2F9)

[Get the /9 wildcard mask →](https://itinsighthub.com/wildcard-mask-calculator/?q=%2F9)

### Related prefixes

[/8](https://itinsighthub.com/subnet/8/)[/10](https://itinsighthub.com/subnet/10/)[/24](https://itinsighthub.com/subnet/24/)[/26](https://itinsighthub.com/subnet/26/)[/28](https://itinsighthub.com/subnet/28/)[/30](https://itinsighthub.com/subnet/30/)

← /8 subnet mask
/10 subnet mask →

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