/20 Subnet
4K IPs — AWS default subnet size
A /20 provides 4,096 addresses. This is the exact size AWS uses for the subnets in the default VPC (one /20 per availability zone). A good choice for a medium-sized availability zone subnet.
Common Use Cases
- → AWS default subnet (per AZ)
- → Medium-large application tier subnet
- → Office floor VLAN
Example /20 Networks
| CIDR | Network | First Usable | Last Usable | Broadcast | Hosts |
|---|---|---|---|---|---|
| 10.0.0.0/20 | 10.0.0.0 | 10.0.0.1 | 10.0.15.254 | 10.0.15.255 | 4,094 |
| 172.16.0.0/20 | 172.16.0.0 | 172.16.0.1 | 172.16.15.254 | 172.16.15.255 | 4,094 |
| 172.31.0.0/20 | 172.31.0.0 | 172.31.0.1 | 172.31.15.254 | 172.31.15.255 | 4,094 |
| 192.168.0.0/20 | 192.168.0.0 | 192.168.0.1 | 192.168.15.254 | 192.168.15.255 | 4,094 |
| 192.168.0.0/20 | 192.168.0.0 | 192.168.0.1 | 192.168.15.254 | 192.168.15.255 | 4,094 |
| 10.0.0.0/20 | 10.0.0.0 | 10.0.0.1 | 10.0.15.254 | 10.0.15.255 | 4,094 |
How /20 Compares
| Prefix | Total IPs | Usable Hosts | Subnet Mask | Typical Use |
|---|---|---|---|---|
| /17 | 32,768 | 32,766 | 255.255.128.0 | Large segment within a /16 |
| /18 | 16,384 | 16,382 | 255.255.192.0 | VPC segment |
| /19 | 8,192 | 8,190 | 255.255.224.0 | — |
| /20 ◀ | 4,096 | 4,094 | 255.255.240.0 | AWS default subnet, medium office VLAN |
| /21 | 2,048 | 2,046 | 255.255.248.0 | Enterprise building / large app tier |
| /22 | 1,024 | 1,022 | 255.255.252.0 | Medium office/application segment |
| /23 | 512 | 510 | 255.255.254.0 | Aggregated /24 pair |
Frequently Asked Questions
How many IPs are in a /20?
A /20 subnet contains 4,096 total IP addresses and 4,094 usable host addresses. The network address (first IP) and broadcast address (last IP) are reserved and cannot be assigned to hosts.
What is the subnet mask for /20?
The subnet mask for a /20 is 255.255.240.0.
In CIDR notation, the prefix length /20 means
the first 20 bits of the address are the network portion.
The wildcard (inverse) mask is 0.0.15.255.
How many /20 subnets fit in a /19?
Exactly 2 — a /19 can be split into two /20 subnets of equal size.