/21 subnet
Mask 255.255.248.0 · 2,046 usable hosts · 2,048 addresses
| Subnet mask | 255.255.248.0 |
|---|---|
| Wildcard mask | 0.0.7.255 |
| Binary mask | 11111111.11111111.11111000.00000000 |
| Network / host bits | 21 / 11 |
| Total addresses | 2,048 |
| Usable hosts | 2,046 |
| Per /8 · /16 · /24 | 8,192 · 32 · – |
| Example | 192.168.10.77/21 → network 192.168.8.0, hosts 192.168.8.1 – 192.168.15.254, broadcast 192.168.15.255 |
Where /21 is used
A /21 is a less common size; it sits between /20 (4,096 addresses) and /22 (1,024 addresses).
Block boundaries: a /21 must start at a multiple of 2,048 addresses. Calculate any specific address in the subnet calculator.
Neighbouring prefixes
Questions
How many hosts in a /21?
2,048 addresses, of which 2,046 are usable (the first is the network address and the last is broadcast).
What is the subnet mask for /21?
255.255.248.0. In binary: 11111111.11111111.11111000.00000000. The wildcard (inverse) mask is 0.0.7.255.