The /17 subnet
A /17 subnet uses the mask 255.255.128.0 and holds 32,768 addresses, 32,766 of them usable.
A /17 sits between a /16 and a /24: 32,768 addresses, 32,766 usable, mask 255.255.128.0. It is a common size for a large flat segment or for aggregating several /24s into one route.
Summarizing contiguous /24s into a /17 keeps routing tables small. The block still fits inside a single broadcast domain if you want it to, though most designs subnet it further.
Below you can see the exact network, broadcast, and host range for any /17 you enter, calculated instantly in your browser.
Calculate a /17 network
Accepts CIDR (192.0.2.0/24), IP + mask (192.0.2.0 255.255.255.0), or a bare IP (defaults to /24).
11000000.00000000.00000010.0000000011111111.11111111.10000000.00000000Example: 192.0.2.0/17
Taking 192.0.2.0/17 as a worked example, the block breaks down like this:
| Network address | 192.0.0.0 |
|---|---|
| Usable range | 192.0.0.1 - 192.0.127.254 |
| Broadcast address | 192.0.127.255 |
| Total addresses | 32,768 |
| Usable hosts | 32,766 |
| Size | 128 class C blocks |
Frequently asked questions
What is the subnet mask for a /17?
A /17 uses the subnet mask 255.255.128.0. Its wildcard mask, used in access lists, is 0.0.127.255.
How many hosts are in a /17 subnet?
A /17 has 32,768 total addresses and 32,766 usable hosts (the total minus the network and broadcast addresses, except for /31 and /32).
What is a /17 used for?
Standard LAN size. It is equivalent to 128 class C blocks.
How do I calculate a /17 network?
Set the host bits to zero for the network address and to one for the broadcast address. The calculator on this page does it for any /17 block you enter.