/18 vs /20 — Subnet Comparison
A /18 subnet is 4× larger than a /20. Every additional bit in the prefix halves the address space — the 2-bit difference between these two means /18 has 22 = 4 times as many addresses.
16K IPs — quarter of a /16
Typical Uses
- →VPC tier segmentation
- →Large-office building network
4K IPs — AWS default subnet size
Typical Uses
- →AWS default subnet (per AZ)
- →Medium-large application tier subnet
- →Office floor VLAN
Key Differences
How 4 /20 Subnets Divide a /18
Example using 10.0.0.0/18 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 |
| 3 | 10.0.32.0/20 | 10.0.32.0 | 10.0.32.1 | 10.0.47.254 | 10.0.47.255 | 4,094 |
| 4 | 10.0.48.0/20 | 10.0.48.0 | 10.0.48.1 | 10.0.63.254 | 10.0.63.255 | 4,094 |
FAQ
What is the difference between /18 and /20?
A /18 has 16,382 usable hosts
and a /20 has 4,094.
The subnet masks differ: /18 uses 255.255.192.0
while /20 uses 255.255.240.0.
Every additional bit in the prefix halves the number of addresses — so the 2-bit gap means
/18 is exactly 4× larger.
How many /20 subnets fit in a /18?
Exactly 4 /20 subnets fit perfectly inside one /18 with no wasted space. To split a /18 into /20s, just increment the last 2 bits of the network address for each new subnet.
Which should I choose?
/18 is typically used for: VPC segment. /20 is better for: AWS default subnet, medium office VLAN. Choose the smallest prefix that comfortably fits your host count — over-allocating wastes address space, but under-allocating means painful renumbering later.