/17 Subnet
32K IPs — half a /16
A /17 splits a /16 into two halves, each with 32,768 addresses. Useful when you need to divide a VPC into two large segments (e.g. public vs. private).
Common Use Cases
- → Half of a /16 VPC split public/private
- → Large campus segment
Example /17 Networks
| CIDR | Network | First Usable | Last Usable | Broadcast | Hosts |
|---|---|---|---|---|---|
| 10.0.0.0/17 | 10.0.0.0 | 10.0.0.1 | 10.0.127.254 | 10.0.127.255 | 32,766 |
| 172.16.0.0/17 | 172.16.0.0 | 172.16.0.1 | 172.16.127.254 | 172.16.127.255 | 32,766 |
| 172.31.0.0/17 | 172.31.0.0 | 172.31.0.1 | 172.31.127.254 | 172.31.127.255 | 32,766 |
| 192.168.0.0/17 | 192.168.0.0 | 192.168.0.1 | 192.168.127.254 | 192.168.127.255 | 32,766 |
| 192.168.0.0/17 | 192.168.0.0 | 192.168.0.1 | 192.168.127.254 | 192.168.127.255 | 32,766 |
| 10.0.0.0/17 | 10.0.0.0 | 10.0.0.1 | 10.0.127.254 | 10.0.127.255 | 32,766 |
How /17 Compares
| Prefix | Total IPs | Usable Hosts | Subnet Mask | Typical Use |
|---|---|---|---|---|
| /14 | 262,144 | 262,142 | 255.252.0.0 | — |
| /15 | 131,072 | 131,070 | 255.254.0.0 | — |
| /16 | 65,536 | 65,534 | 255.255.0.0 | Cloud VPC CIDR, campus network |
| /17 ◀ | 32,768 | 32,766 | 255.255.128.0 | Large segment within a /16 |
| /18 | 16,384 | 16,382 | 255.255.192.0 | VPC segment |
| /19 | 8,192 | 8,190 | 255.255.224.0 | — |
| /20 | 4,096 | 4,094 | 255.255.240.0 | AWS default subnet, medium office VLAN |
Frequently Asked Questions
How many IPs are in a /17?
A /17 subnet contains 32,768 total IP addresses and 32,766 usable host addresses. The network address (first IP) and broadcast address (last IP) are reserved and cannot be assigned to hosts.
What is the subnet mask for /17?
The subnet mask for a /17 is 255.255.128.0.
In CIDR notation, the prefix length /17 means
the first 17 bits of the address are the network portion.
The wildcard (inverse) mask is 0.0.127.255.
How many /17 subnets fit in a /16?
Exactly 2 — a /16 can be split into two /17 subnets of equal size. Each /17 subnet has 32,766 usable hosts, compared to 65,534 in the parent /16.