/17 subnet (255.255.128.0)
A /17 network has 32,768 addresses, of which 32,766 can be given to hosts after the network and broadcast addresses. A large network, the classic “class B” size at /16. Common as a cloud VPC range or a site-wide allocation.
- CIDR notation
- /17
- Subnet mask
- 255.255.128.0
- Wildcard mask
- 0.0.127.255
- Binary mask
- 11111111.11111111.10000000.00000000
- Total addresses
- 32,768
- Usable hosts
- 32,766
- Example network
- 172.16.0.0/17
- Example usable range
- 172.16.0.1 – 172.16.127.254
- /24 networks in a /17
- 128
Calculate a /17 for your own address
In firewall rules and investigations
A /17 rule matches every address from 172.16.0.0 to 172.16.127.255. In Cisco ACLs the same range is written with the wildcard mask 0.0.127.255. When an alert names an IP, checking whether it falls inside a known /17 (your own network, a cloud provider’s published range or a blocked range) is often the quickest first step.