/21 vs /23 — Subnet Comparison
A /21 subnet is 4× larger than a /23. Every additional bit in the prefix halves the address space — the 2-bit difference between these two means /21 has 22 = 4 times as many addresses.
2K IPs — building-scale subnet
Typical Uses
- →Large application tier
- →Enterprise building network
- →Kubernetes node pool subnet
512 IPs — two /24s combined
Typical Uses
- →Two-floor office network
- →Expanded department VLAN
- →Route aggregation of two /24s
Key Differences
How 4 /23 Subnets Divide a /21
Example using 10.0.0.0/21 as the parent block.
| # | CIDR | Network | First Usable | Last Usable | Broadcast | Hosts |
|---|---|---|---|---|---|---|
| 1 | 10.0.0.0/23 | 10.0.0.0 | 10.0.0.1 | 10.0.1.254 | 10.0.1.255 | 510 |
| 2 | 10.0.2.0/23 | 10.0.2.0 | 10.0.2.1 | 10.0.3.254 | 10.0.3.255 | 510 |
| 3 | 10.0.4.0/23 | 10.0.4.0 | 10.0.4.1 | 10.0.5.254 | 10.0.5.255 | 510 |
| 4 | 10.0.6.0/23 | 10.0.6.0 | 10.0.6.1 | 10.0.7.254 | 10.0.7.255 | 510 |
FAQ
What is the difference between /21 and /23?
A /21 has 2,046 usable hosts
and a /23 has 510.
The subnet masks differ: /21 uses 255.255.248.0
while /23 uses 255.255.254.0.
Every additional bit in the prefix halves the number of addresses — so the 2-bit gap means
/21 is exactly 4× larger.
How many /23 subnets fit in a /21?
Exactly 4 /23 subnets fit perfectly inside one /21 with no wasted space. To split a /21 into /23s, just increment the last 2 bits of the network address for each new subnet.
Which should I choose?
/21 is typically used for: Enterprise building / large app tier. /23 is better for: Aggregated /24 pair. Choose the smallest prefix that comfortably fits your host count — over-allocating wastes address space, but under-allocating means painful renumbering later.