/20 vs /21 — Subnet Comparison
A /20 subnet is 2× larger than a /21. Every additional bit in the prefix halves the address space — the 1-bit difference between these two means /20 has 21 = 2 times as many addresses.
4K IPs — AWS default subnet size
Typical Uses
- →AWS default subnet (per AZ)
- →Medium-large application tier subnet
- →Office floor VLAN
2K IPs — building-scale subnet
Typical Uses
- →Large application tier
- →Enterprise building network
- →Kubernetes node pool subnet
Key Differences
How 2 /21 Subnets Divide a /20
Example using 10.0.0.0/20 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 |
FAQ
What is the difference between /20 and /21?
A /20 has 4,094 usable hosts
and a /21 has 2,046.
The subnet masks differ: /20 uses 255.255.240.0
while /21 uses 255.255.248.0.
Every additional bit in the prefix halves the number of addresses — so the 1-bit gap means
/20 is exactly 2× larger.
How many /21 subnets fit in a /20?
Exactly 2 /21 subnets fit perfectly inside one /20 with no wasted space. To split a /20 into /21s, just increment the last 1 bit of the network address for each new subnet.
Which should I choose?
/20 is typically used for: AWS default subnet, medium office VLAN. /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.