/17 vs /21 — Subnet Comparison
A /17 subnet is 16× larger than a /21. Every additional bit in the prefix halves the address space — the 4-bit difference between these two means /17 has 24 = 16 times as many addresses.
32K IPs — half a /16
Typical Uses
- →Half of a /16 VPC split public/private
- →Large campus segment
2K IPs — building-scale subnet
Typical Uses
- →Large application tier
- →Enterprise building network
- →Kubernetes node pool subnet
Key Differences
How 16 /21 Subnets Divide a /17
Example using 10.0.0.0/17 as the parent block.
| # | CIDR | Network | First Usable | Last Usable | Broadcast | Hosts |
|---|---|---|---|---|---|---|
| 1 | 10.0.0.0/21 | 10.0.0.0 | 10.0.0.1 | 10.0.7.254 | 10.0.7.255 | 2,046 |
| 2 | 10.0.8.0/21 | 10.0.8.0 | 10.0.8.1 | 10.0.15.254 | 10.0.15.255 | 2,046 |
| 3 | 10.0.16.0/21 | 10.0.16.0 | 10.0.16.1 | 10.0.23.254 | 10.0.23.255 | 2,046 |
| 4 | 10.0.24.0/21 | 10.0.24.0 | 10.0.24.1 | 10.0.31.254 | 10.0.31.255 | 2,046 |
| 5 | 10.0.32.0/21 | 10.0.32.0 | 10.0.32.1 | 10.0.39.254 | 10.0.39.255 | 2,046 |
| 6 | 10.0.40.0/21 | 10.0.40.0 | 10.0.40.1 | 10.0.47.254 | 10.0.47.255 | 2,046 |
| 7 | 10.0.48.0/21 | 10.0.48.0 | 10.0.48.1 | 10.0.55.254 | 10.0.55.255 | 2,046 |
| 8 | 10.0.56.0/21 | 10.0.56.0 | 10.0.56.1 | 10.0.63.254 | 10.0.63.255 | 2,046 |
| 9 | 10.0.64.0/21 | 10.0.64.0 | 10.0.64.1 | 10.0.71.254 | 10.0.71.255 | 2,046 |
| 10 | 10.0.72.0/21 | 10.0.72.0 | 10.0.72.1 | 10.0.79.254 | 10.0.79.255 | 2,046 |
| 11 | 10.0.80.0/21 | 10.0.80.0 | 10.0.80.1 | 10.0.87.254 | 10.0.87.255 | 2,046 |
| 12 | 10.0.88.0/21 | 10.0.88.0 | 10.0.88.1 | 10.0.95.254 | 10.0.95.255 | 2,046 |
| 13 | 10.0.96.0/21 | 10.0.96.0 | 10.0.96.1 | 10.0.103.254 | 10.0.103.255 | 2,046 |
| 14 | 10.0.104.0/21 | 10.0.104.0 | 10.0.104.1 | 10.0.111.254 | 10.0.111.255 | 2,046 |
| 15 | 10.0.112.0/21 | 10.0.112.0 | 10.0.112.1 | 10.0.119.254 | 10.0.119.255 | 2,046 |
| 16 | 10.0.120.0/21 | 10.0.120.0 | 10.0.120.1 | 10.0.127.254 | 10.0.127.255 | 2,046 |
FAQ
What is the difference between /17 and /21?
A /17 has 32,766 usable hosts
and a /21 has 2,046.
The subnet masks differ: /17 uses 255.255.128.0
while /21 uses 255.255.248.0.
Every additional bit in the prefix halves the number of addresses — so the 4-bit gap means
/17 is exactly 16× larger.
How many /21 subnets fit in a /17?
Exactly 16 /21 subnets fit perfectly inside one /17 with no wasted space. To split a /17 into /21s, just increment the last 4 bits of the network address for each new subnet.
Which should I choose?
/17 is typically used for: Large segment within a /16. /21 is better for: Enterprise building / large app tier. Choose the smallest prefix that comfortably fits your host count — over-allocating wastes address space, but under-allocating means painful renumbering later.