/9 Subnet Mask
A /9 prefix means the subnet mask is 255.128.0.0. It has 23 host bits, giving 8,388,608 total addresses and 8,388,606 usable hosts.
/9 at a glance
| Property | Value |
|---|---|
| CIDR prefix | /9 |
| Subnet mask | 255.128.0.0 |
| Wildcard mask | 0.127.255.255 |
| Total addresses | 8,388,608 |
| Usable hosts | 8,388,606 |
| Host bits | 23 |
How it is calculated
A /9 sets the first 9 bits to 1 (network) and the remaining 23 bits to 0 (host). Usable hosts = 223 − 2 = 8,388,606.
The wildcard mask 0.127.255.255 is the inverse — used in Cisco ACLs and OSPF network statements.
Frequently asked questions
What is a /9 subnet mask?
A /9 equals 255.128.0.0 in dotted decimal (wildcard 0.127.255.255). It provides 8,388,608 total addresses and 8,388,606 usable hosts.
How many hosts are in a /9?
A /9 has 23 host bits, so it supports 8,388,606 usable hosts.
What is the wildcard mask for /9?
The wildcard (inverse) mask for /9 is 0.127.255.255, used in Cisco ACLs and OSPF statements.
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