CIDR prefix
/21
255.255.248.0
2,046 usable hosts · 2,048 addresses
The mask for /21 is 255.255.248.0 — 2,048 addresses, 2,046 usable hosts. Campus and larger office networks; /22–/23 are common for offices with a few hundred devices.
Mask
255.255.248.0
Wildcard
0.0.7.255
Addresses
2,048
Hosts
2,046
Worked example: 192.168.0.0/21
Network address192.168.0.0/21
Broadcast address192.168.7.255
First host192.168.0.1
Last host192.168.7.254
FAQ — /21
What is the subnet mask for /21?
The subnet mask for /21 is 255.255.248.0 (wildcard: 0.0.7.255). The first 21 bits are the network part, the remaining 11 are the host part.
How many hosts fit in a /21?
A /21 block holds 2,048 addresses, of which 2,046 are usable hosts (the network and broadcast addresses are excluded).
How many /21 subnets fit in a /16?
A /16 network divides into 32 /21 subnets.
What is a /21 used for?
Campus and larger office networks; /22–/23 are common for offices with a few hundred devices.