/21 vs /24 — Subnet Comparison
A /21 subnet is 8× larger than a /24. Every additional bit in the prefix halves the address space — the 3-bit difference between these two means /21 has 23 = 8 times as many addresses.
2K IPs — building-scale subnet
Typical Uses
- →Large application tier
- →Enterprise building network
- →Kubernetes node pool subnet
254 usable hosts — the industry standard
Typical Uses
- →Home and SOHO LAN (192.168.1.0/24)
- →Standard office VLAN
- →AWS/Azure subnet per AZ
Key Differences
How 8 /24 Subnets Divide a /21
Example using 10.0.0.0/21 as the parent block.
| # | CIDR | Network | First Usable | Last Usable | Broadcast | Hosts |
|---|---|---|---|---|---|---|
| 1 | 10.0.0.0/24 | 10.0.0.0 | 10.0.0.1 | 10.0.0.254 | 10.0.0.255 | 254 |
| 2 | 10.0.1.0/24 | 10.0.1.0 | 10.0.1.1 | 10.0.1.254 | 10.0.1.255 | 254 |
| 3 | 10.0.2.0/24 | 10.0.2.0 | 10.0.2.1 | 10.0.2.254 | 10.0.2.255 | 254 |
| 4 | 10.0.3.0/24 | 10.0.3.0 | 10.0.3.1 | 10.0.3.254 | 10.0.3.255 | 254 |
| 5 | 10.0.4.0/24 | 10.0.4.0 | 10.0.4.1 | 10.0.4.254 | 10.0.4.255 | 254 |
| 6 | 10.0.5.0/24 | 10.0.5.0 | 10.0.5.1 | 10.0.5.254 | 10.0.5.255 | 254 |
| 7 | 10.0.6.0/24 | 10.0.6.0 | 10.0.6.1 | 10.0.6.254 | 10.0.6.255 | 254 |
| 8 | 10.0.7.0/24 | 10.0.7.0 | 10.0.7.1 | 10.0.7.254 | 10.0.7.255 | 254 |
FAQ
What is the difference between /21 and /24?
A /21 has 2,046 usable hosts
and a /24 has 254.
The subnet masks differ: /21 uses 255.255.248.0
while /24 uses 255.255.255.0.
Every additional bit in the prefix halves the number of addresses — so the 3-bit gap means
/21 is exactly 8× larger.
How many /24 subnets fit in a /21?
Exactly 8 /24 subnets fit perfectly inside one /21 with no wasted space. To split a /21 into /24s, just increment the last 3 bits of the network address for each new subnet.
Which should I choose?
/21 is typically used for: Enterprise building / large app tier. /24 is better for: Standard subnet — home, office, cloud. Choose the smallest prefix that comfortably fits your host count — over-allocating wastes address space, but under-allocating means painful renumbering later.