/9 Subnet
8.3 million IPs — half a /8
A /9 splits a /8 into two equal halves, each with 8,388,608 addresses. Used in very large aggregated routing blocks.
Common Use Cases
- → Large regional ISP allocation
- → Half of a Class A block
- → Aggregated routing prefix
Example /9 Networks
| CIDR | Network | First Usable | Last Usable | Broadcast | Hosts |
|---|---|---|---|---|---|
| 10.0.0.0/9 | 10.0.0.0 | 10.0.0.1 | 10.127.255.254 | 10.127.255.255 | 8,388,606 |
| 172.0.0.0/9 | 172.0.0.0 | 172.0.0.1 | 172.127.255.254 | 172.127.255.255 | 8,388,606 |
| 172.0.0.0/9 | 172.0.0.0 | 172.0.0.1 | 172.127.255.254 | 172.127.255.255 | 8,388,606 |
| 192.128.0.0/9 | 192.128.0.0 | 192.128.0.1 | 192.255.255.254 | 192.255.255.255 | 8,388,606 |
| 192.128.0.0/9 | 192.128.0.0 | 192.128.0.1 | 192.255.255.254 | 192.255.255.255 | 8,388,606 |
| 10.0.0.0/9 | 10.0.0.0 | 10.0.0.1 | 10.127.255.254 | 10.127.255.255 | 8,388,606 |
How /9 Compares
| Prefix | Total IPs | Usable Hosts | Subnet Mask | Typical Use |
|---|---|---|---|---|
| /6 | 67,108,864 | 67,108,862 | 252.0.0.0 | — |
| /7 | 33,554,432 | 33,554,430 | 254.0.0.0 | — |
| /8 | 16,777,216 | 16,777,214 | 255.0.0.0 | Entire private Class A or ISP allocation |
| /9 ◀ | 8,388,608 | 8,388,606 | 255.128.0.0 | Large regional allocations |
| /10 | 4,194,304 | 4,194,302 | 255.192.0.0 | Enterprise regional segment |
| /11 | 2,097,152 | 2,097,150 | 255.224.0.0 | — |
| /12 | 1,048,576 | 1,048,574 | 255.240.0.0 | RFC 1918 Class B range |
Frequently Asked Questions
How many IPs are in a /9?
A /9 subnet contains 8,388,608 total IP addresses and 8,388,606 usable host addresses. The network address (first IP) and broadcast address (last IP) are reserved and cannot be assigned to hosts.
What is the subnet mask for /9?
The subnet mask for a /9 is 255.128.0.0.
In CIDR notation, the prefix length /9 means
the first 9 bits of the address are the network portion.
The wildcard (inverse) mask is 0.127.255.255.
How many /9 subnets fit in a /8?
Exactly 2 — a /8 can be split into two /9 subnets of equal size. Each /9 subnet has 8,388,606 usable hosts, compared to 16,777,214 in the parent /8.