/19 vs /21 — Subnet Comparison
A /19 subnet is 4× larger than a /21. Every additional bit in the prefix halves the address space — the 2-bit difference between these two means /19 has 22 = 4 times as many addresses.
2K IPs — building-scale subnet
Typical Uses
- →Large application tier
- →Enterprise building network
- →Kubernetes node pool subnet
Key Differences
How 4 /21 Subnets Divide a /19
Example using 10.0.0.0/19 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 |
FAQ
What is the difference between /19 and /21?
A /19 has 8,190 usable hosts
and a /21 has 2,046.
The subnet masks differ: /19 uses 255.255.224.0
while /21 uses 255.255.248.0.
Every additional bit in the prefix halves the number of addresses — so the 2-bit gap means
/19 is exactly 4× larger.
How many /21 subnets fit in a /19?
Exactly 4 /21 subnets fit perfectly inside one /19 with no wasted space. To split a /19 into /21s, just increment the last 2 bits of the network address for each new subnet.
Which should I choose?
Choose based on how many hosts you need. Use the hosts → prefix calculator on the homepage to find the right size for your requirements.