/21 Subnet
2K IPs — building-scale subnet
A /21 subnet provides 2,048 IP addresses (2,046 usable). Suitable for a medium enterprise segment or a large application tier with room to grow.
Common Use Cases
- → Large application tier
- → Enterprise building network
- → Kubernetes node pool subnet
Example /21 Networks
| CIDR | Network | First Usable | Last Usable | Broadcast | Hosts |
|---|---|---|---|---|---|
| 10.0.0.0/21 | 10.0.0.0 | 10.0.0.1 | 10.0.7.254 | 10.0.7.255 | 2,046 |
| 172.16.0.0/21 | 172.16.0.0 | 172.16.0.1 | 172.16.7.254 | 172.16.7.255 | 2,046 |
| 172.31.0.0/21 | 172.31.0.0 | 172.31.0.1 | 172.31.7.254 | 172.31.7.255 | 2,046 |
| 192.168.0.0/21 | 192.168.0.0 | 192.168.0.1 | 192.168.7.254 | 192.168.7.255 | 2,046 |
| 192.168.0.0/21 | 192.168.0.0 | 192.168.0.1 | 192.168.7.254 | 192.168.7.255 | 2,046 |
| 10.0.0.0/21 | 10.0.0.0 | 10.0.0.1 | 10.0.7.254 | 10.0.7.255 | 2,046 |
How /21 Compares
| Prefix | Total IPs | Usable Hosts | Subnet Mask | Typical Use |
|---|---|---|---|---|
| /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 |
| /24 | 256 | 254 | 255.255.255.0 | Standard subnet — home, office, cloud |
Frequently Asked Questions
How many IPs are in a /21?
A /21 subnet contains 2,048 total IP addresses and 2,046 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 /21?
The subnet mask for a /21 is 255.255.248.0.
In CIDR notation, the prefix length /21 means
the first 21 bits of the address are the network portion.
The wildcard (inverse) mask is 0.0.7.255.
How many /21 subnets fit in a /20?
Exactly 2 — a /20 can be split into two /21 subnets of equal size. Each /21 subnet has 2,046 usable hosts, compared to 4,094 in the parent /20.