/16 vs /17 — Subnet Comparison
A /16 subnet is 2× larger than a /17. Every additional bit in the prefix halves the address space — the 1-bit difference between these two means /16 has 21 = 2 times as many addresses.
65K IPs — standard VPC or site block
Typical Uses
- →AWS / Azure VPC or VNet CIDR
- →Enterprise campus or data centre segment
- →Docker overlay network
32K IPs — half a /16
Typical Uses
- →Half of a /16 VPC split public/private
- →Large campus segment
Key Differences
How 2 /17 Subnets Divide a /16
Example using 10.0.0.0/16 as the parent block.
| # | CIDR | Network | First Usable | Last Usable | Broadcast | Hosts |
|---|---|---|---|---|---|---|
| 1 | 10.0.0.0/17 | 10.0.0.0 | 10.0.0.1 | 10.0.127.254 | 10.0.127.255 | 32,766 |
| 2 | 10.0.128.0/17 | 10.0.128.0 | 10.0.128.1 | 10.0.255.254 | 10.0.255.255 | 32,766 |
FAQ
What is the difference between /16 and /17?
A /16 has 65,534 usable hosts
and a /17 has 32,766.
The subnet masks differ: /16 uses 255.255.0.0
while /17 uses 255.255.128.0.
Every additional bit in the prefix halves the number of addresses — so the 1-bit gap means
/16 is exactly 2× larger.
How many /17 subnets fit in a /16?
Exactly 2 /17 subnets fit perfectly inside one /16 with no wasted space. To split a /16 into /17s, just increment the last 1 bit of the network address for each new subnet.
Which should I choose?
/16 is typically used for: Cloud VPC CIDR, campus network. /17 is better for: Large segment within a /16. Choose the smallest prefix that comfortably fits your host count — over-allocating wastes address space, but under-allocating means painful renumbering later.