/15 vs /17 — Subnet Comparison
A /15 subnet is 4× larger than a /17. Every additional bit in the prefix halves the address space — the 2-bit difference between these two means /15 has 22 = 4 times as many addresses.
32K IPs — half a /16
Typical Uses
- →Half of a /16 VPC split public/private
- →Large campus segment
Key Differences
How 4 /17 Subnets Divide a /15
Example using 10.0.0.0/15 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 |
| 3 | 10.1.0.0/17 | 10.1.0.0 | 10.1.0.1 | 10.1.127.254 | 10.1.127.255 | 32,766 |
| 4 | 10.1.128.0/17 | 10.1.128.0 | 10.1.128.1 | 10.1.255.254 | 10.1.255.255 | 32,766 |
FAQ
What is the difference between /15 and /17?
A /15 has 131,070 usable hosts
and a /17 has 32,766.
The subnet masks differ: /15 uses 255.254.0.0
while /17 uses 255.255.128.0.
Every additional bit in the prefix halves the number of addresses — so the 2-bit gap means
/15 is exactly 4× larger.
How many /17 subnets fit in a /15?
Exactly 4 /17 subnets fit perfectly inside one /15 with no wasted space. To split a /15 into /17s, just increment the last 2 bits of the network address for each new subnet.
Which should I choose?
Choose based on how many hosts you need. Use the hosts → prefix calculator on the homepage to find the right size for your requirements.