The /21 subnet
A /21 subnet uses the mask 255.255.248.0 and holds 2,048 addresses, 2,046 of them usable.
A /21 sits between a /16 and a /24: 2,048 addresses, 2,046 usable, mask 255.255.248.0. It is a common size for a large flat segment or for aggregating several /24s into one route.
Summarizing contiguous /24s into a /21 keeps routing tables small. The block still fits inside a single broadcast domain if you want it to, though most designs subnet it further.
Below you can see the exact network, broadcast, and host range for any /21 you enter, calculated instantly in your browser.
Calculate a /21 network
Accepts CIDR (192.0.2.0/24), IP + mask (192.0.2.0 255.255.255.0), or a bare IP (defaults to /24).
11000000.00000000.00000010.0000000011111111.11111111.11111000.00000000Example: 192.0.2.0/21
Taking 192.0.2.0/21 as a worked example, the block breaks down like this:
| Network address | 192.0.0.0 |
|---|---|
| Usable range | 192.0.0.1 - 192.0.7.254 |
| Broadcast address | 192.0.7.255 |
| Total addresses | 2,048 |
| Usable hosts | 2,046 |
| Size | 8 class C blocks |
Frequently asked questions
What is the subnet mask for a /21?
A /21 uses the subnet mask 255.255.248.0. Its wildcard mask, used in access lists, is 0.0.7.255.
How many hosts are in a /21 subnet?
A /21 has 2,048 total addresses and 2,046 usable hosts (the total minus the network and broadcast addresses, except for /31 and /32).
What is a /21 used for?
Standard LAN size. It is equivalent to 8 class C blocks.
How do I calculate a /21 network?
Set the host bits to zero for the network address and to one for the broadcast address. The calculator on this page does it for any /21 block you enter.