Subnet Mask
/17 Subnet Mask
A /17 prefix means the subnet mask is 255.255.128.0. It has 15 host bits, giving 32,768 total addresses and 32,766 usable hosts.
/17 at a glance
| Property | Value |
|---|---|
| CIDR prefix | /17 |
| Subnet mask | 255.255.128.0 |
| Wildcard mask | 0.0.127.255 |
| Total addresses | 32,768 |
| Usable hosts | 32,766 |
| Host bits | 15 |
How it is calculated
A /17 sets the first 17 bits to 1 (network) and the remaining 15 bits to 0 (host). Usable hosts = 215 − 2 = 32,766.
The wildcard mask 0.0.127.255 is the inverse — used in Cisco ACLs and OSPF network statements.
Frequently asked questions
What is a /17 subnet mask?
A /17 equals 255.255.128.0 in dotted decimal (wildcard 0.0.127.255). It provides 32,768 total addresses and 32,766 usable hosts.
How many hosts are in a /17?
A /17 has 15 host bits, so it supports 32,766 usable hosts.
What is the wildcard mask for /17?
The wildcard (inverse) mask for /17 is 0.0.127.255, used in Cisco ACLs and OSPF statements.
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