/19 vs /20 — Subnet Comparison
A /19 subnet is 2× larger than a /20. Every additional bit in the prefix halves the address space — the 1-bit difference between these two means /19 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
Key Differences
How 2 /20 Subnets Divide a /19
Example using 10.0.0.0/19 as the parent block.
| # | CIDR | Network | First Usable | Last Usable | Broadcast | Hosts |
|---|---|---|---|---|---|---|
| 1 | 10.0.0.0/20 | 10.0.0.0 | 10.0.0.1 | 10.0.15.254 | 10.0.15.255 | 4,094 |
| 2 | 10.0.16.0/20 | 10.0.16.0 | 10.0.16.1 | 10.0.31.254 | 10.0.31.255 | 4,094 |
FAQ
What is the difference between /19 and /20?
A /19 has 8,190 usable hosts
and a /20 has 4,094.
The subnet masks differ: /19 uses 255.255.224.0
while /20 uses 255.255.240.0.
Every additional bit in the prefix halves the number of addresses — so the 1-bit gap means
/19 is exactly 2× larger.
How many /20 subnets fit in a /19?
Exactly 2 /20 subnets fit perfectly inside one /19 with no wasted space. To split a /19 into /20s, just increment the last 1 bit 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.