Detailed information about IPv4 /21 subnets
A /21 network provides thousands of usable IP addresses, making it suitable for medium to large organizations or campus networks.
| CIDR Notation | /21 |
| Subnet Mask | 255.255.248.0 |
| Binary Subnet Mask | 11111111 11111111 11111000 00000000 |
| Total Addresses | 2,048 |
| Usable Hosts (Standard) | 2,046 |
| AWS Usable IPs | 2,041 |
| Azure Usable IPs | 2,041 |
| GCP Usable IPs | 2,042 |
A CIDR /21 subnet has 2,046 usable IP addresses for hosts. This is a relatively large subnet that can be further divided into smaller subnets if needed.
This subnet size strikes a balance between address space efficiency and management overhead.
Cloud providers reserve some IP addresses in each subnet for internal use, resulting in fewer usable IPs:
When planning your cloud infrastructure, consider these reserved addresses to accurately size your subnets.