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/17 Subnet Mask: 255.255.128.0

A /17 prefix fixes the first 17 bits of the address as the network, leaving 15 bits for hosts. That gives 32,768 addresses, of which 32,766 can be assigned to hosts.

CIDR notation/17
Subnet mask255.255.128.0
Wildcard mask0.0.127.255
Binary mask11111111.11111111.10000000.00000000
Hex mask0xFFFF8000
Total addresses32,768
Usable hosts32,766

Example: 172.16.0.0/17

  • Network address: 172.16.0.0
  • First usable host: 172.16.0.1
  • Last usable host: 172.16.127.254
  • Broadcast address: 172.16.127.255

When to use a /17 subnet

Entire VPCs or campus networks. /16 is the largest CIDR block AWS allows for a VPC.

  • 512 /17 subnets fit in a /8.
  • 2 /17 subnets fit in a /16.

Frequently Asked Questions

What is the subnet mask for /17?

The subnet mask for /17 is 255.255.128.0, and its wildcard (inverse) mask is 0.0.127.255.

How many hosts are in a /17?

A /17 contains 32,768 addresses. Subtracting the network and broadcast addresses leaves 32,766 usable hosts.