/21 vs /22 — Subnet Comparison
A /21 subnet is 2× larger than a /22. Every additional bit in the prefix halves the address space — the 1-bit difference between these two means /21 has 21 = 2 times as many addresses.
2K IPs — building-scale subnet
Typical Uses
- →Large application tier
- →Enterprise building network
- →Kubernetes node pool subnet
1K IPs — medium site subnet
Typical Uses
- →Medium office floor VLAN
- →Application tier with ~500 hosts
- →Cloud subnet for a single microservice cluster
Key Differences
How 2 /22 Subnets Divide a /21
Example using 10.0.0.0/21 as the parent block.
| # | CIDR | Network | First Usable | Last Usable | Broadcast | Hosts |
|---|---|---|---|---|---|---|
| 1 | 10.0.0.0/22 | 10.0.0.0 | 10.0.0.1 | 10.0.3.254 | 10.0.3.255 | 1,022 |
| 2 | 10.0.4.0/22 | 10.0.4.0 | 10.0.4.1 | 10.0.7.254 | 10.0.7.255 | 1,022 |
FAQ
What is the difference between /21 and /22?
A /21 has 2,046 usable hosts
and a /22 has 1,022.
The subnet masks differ: /21 uses 255.255.248.0
while /22 uses 255.255.252.0.
Every additional bit in the prefix halves the number of addresses — so the 1-bit gap means
/21 is exactly 2× larger.
How many /22 subnets fit in a /21?
Exactly 2 /22 subnets fit perfectly inside one /21 with no wasted space. To split a /21 into /22s, just increment the last 1 bit of the network address for each new subnet.
Which should I choose?
/21 is typically used for: Enterprise building / large app tier. /22 is better for: Medium office/application segment. Choose the smallest prefix that comfortably fits your host count — over-allocating wastes address space, but under-allocating means painful renumbering later.