/21 subnet (255.255.248.0)
A /21 network has 2,048 addresses, of which 2,046 can be given to hosts after the network and broadcast addresses. Several /24s grouped together. Common for larger office networks, cloud VPC subnets and ISP allocations to businesses.
- CIDR notation
- /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 network
- 172.16.0.0/21
- Example usable range
- 172.16.0.1 – 172.16.7.254
- /24 networks in a /21
- 8
Calculate a /21 for your own address
In firewall rules and investigations
A /21 rule matches every address from 172.16.0.0 to 172.16.7.255. In Cisco ACLs the same range is written with the wildcard mask 0.0.7.255. When an alert names an IP, checking whether it falls inside a known /21 (your own network, a cloud provider’s published range or a blocked range) is often the quickest first step.