/9 Subnet Mask ReferenceIPv4
Slice a /8 down the middle and you hold a /9: mask <code>255.128.0.0</code>, 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.
/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: <code>10.128.0.0/9</code> is precisely the upper half of <code>10.0.0.0/8</code>, spanning 10.128.0.0 through 10.255.255.255, with <code>10.0.0.0/9</code> as the lower half.
Frequently asked questions
What is the subnet mask for a /9?
<code>255.128.0.0</code>, wildcard <code>0.127.255.255</code>. 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 <code>x.0.0.0/9</code> and promote the upper half to <code>x.128.0.0/9</code>. 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