The /21 subnet, explained
Everything about a /21 CIDR block — mask, address count, usable hosts and where a /21 fits in real network plans.
| Prefix length | /21 — 21 network bits, 11 host bits |
| Subnet mask | 255.255.248.0 |
| Wildcard (ACL) mask | 0.0.7.255 |
| Binary mask | 11111111.11111111.11111000.00000000 |
| Total addresses | 2^11 = 2,048 |
| Usable hosts | 2,046 (total − network − broadcast) |
| Contains | 8 × /24 subnets |
| A /16 holds | 32 × /21 subnets |
| Example block | 10.10.0.0/21 |
Notes
A /21 prefix means the first 21 bits of the 32-bit IPv4 address identify the network and the remaining 11 bits identify hosts — giving 2^11 = 2,048 addresses. The first address (network) and the last (broadcast) are not assignable in a standard subnet.
Cloud sizing: a /21 is valid as an AWS VPC (allowed range /16–/28). AWS reserves 5 addresses per subnet, so a /21 subnet gives 2,043 assignable IPs there; GCP reserves 4 (2,044 assignable).
Adjacent sizes: /8 · /9 · /10 · /11 · /12 · /13 · /14 · /15 · /16 · /17 · /18 · /19 · /20 · /22 · /23 · /24 · /25 · /26 · /27 · /28 · /29 · /30 · /31 · /32
Frequently asked questions
How many IP addresses are in a /21?
2,048 total addresses, of which 2,046 are usable hosts (network and broadcast addresses are reserved).
What is the subnet mask for /21?
255.255.248.0 (wildcard mask 0.0.7.255).
What does /21 mean in an IP address?
The /21 suffix is CIDR notation: the first 21 bits are the network part, the last 11 bits are the host part.