/17 vs /18 — Subnet Comparison
A /17 subnet is 2× larger than a /18. Every additional bit in the prefix halves the address space — the 1-bit difference between these two means /17 has 21 = 2 times as many addresses.
32K IPs — half a /16
Typical Uses
- →Half of a /16 VPC split public/private
- →Large campus segment
16K IPs — quarter of a /16
Typical Uses
- →VPC tier segmentation
- →Large-office building network
Key Differences
How 2 /18 Subnets Divide a /17
Example using 10.0.0.0/17 as the parent block.
| # | CIDR | Network | First Usable | Last Usable | Broadcast | Hosts |
|---|---|---|---|---|---|---|
| 1 | 10.0.0.0/18 | 10.0.0.0 | 10.0.0.1 | 10.0.63.254 | 10.0.63.255 | 16,382 |
| 2 | 10.0.64.0/18 | 10.0.64.0 | 10.0.64.1 | 10.0.127.254 | 10.0.127.255 | 16,382 |
FAQ
What is the difference between /17 and /18?
A /17 has 32,766 usable hosts
and a /18 has 16,382.
The subnet masks differ: /17 uses 255.255.128.0
while /18 uses 255.255.192.0.
Every additional bit in the prefix halves the number of addresses — so the 1-bit gap means
/17 is exactly 2× larger.
How many /18 subnets fit in a /17?
Exactly 2 /18 subnets fit perfectly inside one /17 with no wasted space. To split a /17 into /18s, just increment the last 1 bit of the network address for each new subnet.
Which should I choose?
/17 is typically used for: Large segment within a /16. /18 is better for: VPC segment. Choose the smallest prefix that comfortably fits your host count — over-allocating wastes address space, but under-allocating means painful renumbering later.