Subnet Mask
/21 Subnet Mask
A /21 prefix means the subnet mask is 255.255.248.0. It has 11 host bits, giving 2,048 total addresses and 2,046 usable hosts.
/21 at a glance
| Property | Value |
|---|---|
| CIDR prefix | /21 |
| Subnet mask | 255.255.248.0 |
| Wildcard mask | 0.0.7.255 |
| Total addresses | 2,048 |
| Usable hosts | 2,046 |
| Host bits | 11 |
How it is calculated
A /21 sets the first 21 bits to 1 (network) and the remaining 11 bits to 0 (host). Usable hosts = 211 − 2 = 2,046.
The wildcard mask 0.0.7.255 is the inverse — used in Cisco ACLs and OSPF network statements.
Frequently asked questions
What is a /21 subnet mask?
A /21 equals 255.255.248.0 in dotted decimal (wildcard 0.0.7.255). It provides 2,048 total addresses and 2,046 usable hosts.
How many hosts are in a /21?
A /21 has 11 host bits, so it supports 2,046 usable hosts.
What is the wildcard mask for /21?
The wildcard (inverse) mask for /21 is 0.0.7.255, used in Cisco ACLs and OSPF statements.
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