/17 subnet mask (255.255.128.0)

A /17 network has 15 host bits, 32,768 addresses and 32,766 usable hosts. It is 128 /24 networks.

/17 at a glance

Netmask: 255.255.128.0
Wildcard mask: 0.0.127.255
Hex mask: 0xFFFF8000
Binary mask: 11111111.11111111.10000000.00000000
Host bits: 15
Total addresses: 32,768
Usable hosts: 32,766 (minus network and broadcast)
Compared with a /24: 128 /24 networks

Calculate a /17 in the subnet calculator

Typical uses of a /17

A /17 is half a /16. Splitting a /16 VPC into a /17 for private subnets and a /17 for public or reserved space is a common AWS pattern.

Example /17 ranges

Network	Network address	First usable	Last usable	Broadcast	Next /17
10.0.0.0/17	10.0.0.0	10.0.0.1	10.0.127.254	10.0.127.255	10.0.128.0/17	calculate
172.16.0.0/17	172.16.0.0	172.16.0.1	172.16.127.254	172.16.127.255	172.16.128.0/17	calculate
192.168.0.0/17	192.168.0.0	192.168.0.1	192.168.127.254	192.168.127.255	192.168.128.0/17	calculate

The /17 boundary falls in the third octet, so /17 networks start every 128 in that octet (2 per /16): 0, 128.

How many /17s fit

In a	Number of /17 subnets
/8	512
/16	2

What a /17 contains

Smaller block	How many
/24	128
/26	512
/28	2,048
/29	4,096
/30	8,192
/32	32,768

Splitting a /17

Into	Subnets	Usable hosts each
/18	2	16,382
/19	4	8,190
/20	8	4,094
/21	16	2,046

/17 in AWS, Azure and Google Cloud

Platform	Allowed subnet sizes	/17 allowed?	Usable after reservation
AWS VPC	/16 to /28	Yes	32,763 (5 reserved)
Azure VNet	/2 to /29	Yes	32,763 (5 reserved)
Google Cloud VPC	/4 to /29	Yes	32,764 (4 reserved)

Cloud providers reserve addresses in every subnet (first four and the last address (network, VPC router, DNS, future use, broadcast) on AWS), so the usable count there differs from the classic 2^n - 2.