A /21 has the mask 255.255.248.0 and 2,046 usable hosts, eight /24s glued together. The third octet steps in blocks of 8. It’s a typical size for a large wireless network on a campus or at an event, where many devices join and leave and DHCP needs a big pool.
Facts
- Prefix
- /21
- Subnet mask
- 255.255.248.0
- Wildcard mask
- 0.0.7.255
- Binary mask
- 11111111.11111111.11111000.00000000
- Total addresses
- 2,048
- Usable hosts
- 2,046
- Example range
- 192.168.0.0/21: 192.168.0.0 – 192.168.7.255
- Typical use
- Campus networks.
Try it
Try: , ,
- CIDR block
- 192.168.0.0/21
- IP range
- 192.168.0.0 – 192.168.7.255
- Addresses
- 2,048
- Usable hosts
- 2,046
- Subnet mask
- 255.255.248.0
- Wildcard mask
- 0.0.7.255
Frequently asked questions
How many hosts are in a /21?
A /21 has 2,048 addresses. Subtracting the network and broadcast addresses leaves 2,046 usable hosts.
What is the subnet mask for /21?
/21 is 255.255.248.0 (wildcard mask 0.0.7.255). In binary that’s 21 ones followed by 11 zeros.