What is a /21 subnet?
2,046 usable hosts out of 2,048 addresses, behind netmask 255.255.248.0.
| Prefix | /21 |
|---|---|
| Netmask | 255.255.248.0 |
| Wildcard | 0.0.7.255 |
| Total addresses | 2,048 |
| Usable hosts | 2,046 |
| Example block | 10.0.0.0/21 |
| Usable host range | 10.0.0.1 – 10.0.7.254 |
| Broadcast | 10.0.7.255 |
A /21 fixes the first 21 bits of the address as the network and leaves 11 for hosts — 2,048 addresses in all.
Of those, 2,046 can be given to a machine: the first address names the network itself, and the last is the broadcast address.
A comfortable size for a large subnet or a small site — more than a single /24, without committing a whole /16.
It is 8 times the size of a /24.
Other subnet sizes
Subnet CalculatorStart in seconds
Bring your whole infrastructure into one modern dashboard.
Free plan · No credit card required · Set up in minutes